Followers
DataBase (DBMS)
1. What is database?
A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose.
2. What is DBMS?
It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.
3. What is a Database system?
The database and DBMS software together is called as Database system.
4. Advantages of DBMS?
? Redundancy is controlled.
? Unauthorised access is restricted.
? Providing multiple user interfaces.
? Enforcing integrity constraints.
? Providing backup and recovery.
5. Disadvantage in File Processing System?
? Data redundancy & inconsistency.
? Difficult in accessing data.
? Data isolation.
? Data integrity.
? Concurrent access is not possible.
? Security Problems.
6. Describe the three levels of data abstraction?
The are three levels of abstraction:
? Physical level: The lowest level of abstraction describes how data are stored.
? Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.
? View level: The highest level of abstraction describes only part of entire database.
7. Define the "integrity rules"
There are two Integrity rules.
? Entity Integrity: States that “Primary key cannot have NULL value”
? Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation.
8. What is extension and intension?
Extension -
It is the number of tuples present in a table at any instance. This is time dependent.
Intension -
It is a constant value that gives the name, structure of table and the constraints laid on it.
9. What is System R? What are its two major subsystems?
System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.
Its two subsystems are
? Research Storage
? System Relational Data System.
10. How is the data structure of System R different from the relational structure?
Unlike Relational systems in System R
? Domains are not supported
? Enforcement of candidate key uniqueness is optional
? Enforcement of entity integrity is optional
? Referential integrity is not enforced
11. What is Data Independence?
Data independence means that “the application is independent of the storage structure and access strategy of data”. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.
Two types of Data Independence:
? Physical Data Independence: Modification in physical level should not affect the logical level.
? Logical Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence is more difficult to achieve
12. What is a view? How it is related to data independence?
A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence.
13. What is Data Model?
A collection of conceptual tools for describing data, data relationships data semantics and constraints.
14. What is E-R model?
This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes.
15. What is Object Oriented model?
This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes.
16. What is an Entity?
It is a 'thing' in the real world with an independent existence.
17. What is an Entity type?
It is a collection (set) of entities that have same attributes.
18. What is an Entity set?
It is a collection of all entities of particular entity type in the database.
19. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped together into an entity set.
20. What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.
21. What is an attribute?
It is a particular property, which describes the entity.
22. What is a Relation Schema and a Relation?
A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn).
23. What is degree of a Relation?
It is the number of attribute of its relation schema.
24. What is Relationship?
It is an association among two or more entities.
25. What is Relationship set?
The collection (or set) of similar relationships.
26. What is Relationship type?
Relationship type defines a set of associations or a relationship set among a given set of entity types.
27. What is degree of Relationship type?
It is the number of entity type participating.
25. What is DDL (Data Definition Language)?
A data base schema is specifies by a set of definitions expressed by a special language called DDL.
26. What is VDL (View Definition Language)?
It specifies user views and their mappings to the conceptual schema.
27. What is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may specify the mapping between two schemas.
28. What is Data Storage - Definition Language?
The storage structures and access methods used by database system are specified by a set of definition in a special type of DDL called data storage-definition language.
29. What is DML (Data Manipulation Language)?
This language that enable user to access or manipulate data as organised by appropriate data model.
? Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.
? Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data.
31. What is DML Compiler?
It translates DML statements in a query language into low-level instruction that the query evaluation engine can understand.
32. What is Query evaluation engine?
It executes low-level instruction generated by compiler.
33. What is DDL Interpreter?
It interprets DDL statements and record them in tables containing metadata.
34. What is Record-at-a-time?
The Low level or Procedural DML can specify and retrieve each record from a set of records. This retrieve of a record is said to be Record-at-a-time.
35. What is Set-at-a-time or Set-oriented?
The High level or Non-procedural DML can specify and retrieve many records in a single DML statement. This retrieve of a record is said to be Set-at-a-time or Set-oriented.
36. What is Relational Algebra?
It is procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation.
37. What is Relational Calculus?
It is an applied predicate calculus specifically tailored for relational databases proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.
38. How does Tuple-oriented relational calculus differ from domain-oriented relational calculus
The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables that range over the underlying domains instead of over relation. E.g. ILL, DEDUCE.
39. What is normalization?
It is a process of analysing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties
? Minimizing redundancy
? Minimizing insertion, deletion and update anomalies.
40. What is Functional Dependency?
A Functional dependency is denoted by X Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely determines the value of component Y.
41. When is a functional dependency F said to be minimal?
? Every dependency in F has a single attribute for its right hand side.
? We cannot replace any dependency X A in F with a dependency Y A where Y is a proper subset of X and still have a set of dependency that is equivalent to F.
? We cannot remove any dependency from F and still have set of dependency that is equivalent to F.
42. What is Multivalued dependency?
Multivalued dependency denoted by X Y specified on relation schema R, where X and Y are both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the following properties
? t3[x] = t4[X] = t1[X] = t2[X]
? t3[Y] = t1[Y] and t4[Y] = t2[Y]
? t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
43. What is Lossless join property?
It guarantees that the spurious tuple generation does not occur with respect to relation schemas after decomposition.
44. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple, indivisible) values.
45. What is Fully Functional dependency?
It is based on concept of full functional dependency. A functional dependency X Y is full functional dependency if removal of any attribute A from X means that the dependency does not hold any more.
46. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.
47. What is 3NF?
A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the following is true
? X is a Super-key of R.
? A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively dependent on primary key.
48. What is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every FD X A, X must be a candidate key.
49. What is 4NF?
A relation schema R is said to be in 4NF if for every Multivalued dependency X Y that holds over R, one of following is true
? X is subset or equal to (or) XY = R.
? X is a super key.
50. What is 5NF?
A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ..., Rn} that holds R, one the following is true
? Ri = R for some i.
? The join dependency is implied by the set of FD, over R in which the left side is key of R.
51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies that should hold on the the constraint can be enforced by simply enforcing the domain constraint and key constraint on the relation.
52. What are partial, alternate,, artificial, compound and natural key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometime called as Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available, then the last resort is to simply create a key, by assigning a unique number to each record or occurrence. Then this is known as developing an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a construct, then combining multiple elements to create a unique identifier for the construct is known as creating a compound key.
Natural Key:
When one of the data elements stored within a construct is utilized as the primary key, then it is called the natural key.
53. What is indexing and what are the different kinds of indexing?
Indexing is a technique for determining how quickly specific data can be found.
Types:
? Binary search style indexing
? B-Tree indexing
? Inverted list indexing
? Memory resident table
? Table indexing
54. What is system catalog or catalog relation? How is better known as?
A RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called data dictionary.
55. What is meant by query optimization?
The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.
56. What is join dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued dependency.A JD {R1, R2, ..., Rn} is said to hold over a relation R if R1, R2, R3, ..., Rn is a lossless-join decomposition of R . There is no set of sound and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns of a relation are contained in other columns. A foreign key constraint is an example of inclusion dependency.
57. What is durability in DBMS?
Once the DBMS informs the user that a transaction has successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. This property is called durability.
58. What do you mean by atomicity and aggregation?
Atomicity:
Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.
59. What is a Phantom Deadlock?
In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.
60. What is a checkpoint and When does it occur?
A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce the amount of work to be done during restart in the event of subsequent crashes.
61. What are the different phases of transaction?
Different phases are
? Analysis phase
? Redo Phase
? Undo phase
62. What do you mean by flat file database?
It is a database in which there are no programs or user access languages. It has no cross-file capabilities but is user-friendly and provides user-interface management.
63. What is "transparent DBMS"?
It is one, which keeps its Physical Structure hidden from user.
64. Brief theory of Network, Hierarchical schemas and their properties
Network schema uses a graph data structure to organize records example for such a database management system is CTCG while a hierarchical schema uses a tree data structure example for such a system is IMS.
65. What is a query?
A query with respect to DBMS relates to user commands that are used to interact with a data base. The query language can be classified into data definition language and data manipulation language.
66. What do you mean by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.
A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.
E.g. Select * From CUST Where '10/03/1990' IN (Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)
67. What are the primitive operations common to all record management systems?
Addition, deletion and modification.
68. Name the buffer in which all the commands that are typed in are stored
‘Edit’ Buffer
69. What are the unary operations in Relational Algebra?
PROJECTION and SELECTION.
70. Are the resulting relations of PRODUCT and JOIN operation the same?
No.
PRODUCT: Concatenation of every row in one relation with every row in another.
JOIN: Concatenation of rows from one relation and related rows from another.
71. What is RDBMS KERNEL?
Two important pieces of RDBMS architecture are the kernel, which is the software, and the data dictionary, which consists of the system-level data structures used by the kernel to manage the database
You might think of an RDBMS as an operating system (or set of subsystems), designed specifically for controlling data access; its primary functions are storing, retrieving, and securing data. An RDBMS maintains its own list of authorized users and their associated privileges; manages memory caches and paging; controls locking for concurrent resource usage; dispatches and schedules user requests; and manages space usage within its table-space structures
.
72. Name the sub-systems of a RDBMS
I/O, Security, Language Processing, Process Control, Storage Management, Logging and Recovery, Distribution Control, Transaction Control, Memory Management, Lock Management
73. Which part of the RDBMS takes care of the data dictionary? How
Data dictionary is a set of tables and database objects that is stored in a special area of the database and maintained exclusively by the kernel.
74. What is the job of the information stored in data-dictionary?
The information in the data dictionary validates the existence of the objects, provides access to them, and maps the actual physical storage location.
75. Not only RDBMS takes care of locating data it also
determines an optimal access path to store or retrieve the data
76. How do you communicate with an RDBMS?
You communicate with an RDBMS using Structured Query Language (SQL)
77. Define SQL and state the differences between SQL and other conventional programming Languages
SQL is a nonprocedural language that is designed specifically for data access operations on normalized relational database structures. The primary difference between SQL and other conventional programming languages is that SQL statements specify what data operations should be performed rather than how to perform them.
78. Name the three major set of files on disk that compose a database in Oracle
There are three major sets of files on disk that compose a database. All the files are binary. These are
? Database files
? Control files
? Redo logs
The most important of these are the database files where the actual data resides. The control files and the redo logs support the functioning of the architecture itself.
All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a backup, if there is one.
79. What is an Oracle Instance?
The Oracle system processes, also known as Oracle background processes, provide functions for the user processes—functions that would otherwise be done by the user processes themselves
Oracle database-wide system memory is known as the SGA, the system global area or shared global area. The data and control structures in the SGA are shareable, and all the Oracle background processes and user processes can use them.
The combination of the SGA and the Oracle background processes is known as an Oracle instance
80. What are the four Oracle system processes that must always be up and running for the database to be useable
The four Oracle system processes that must always be up and running for the database to be useable include DBWR (Database Writer), LGWR (Log Writer), SMON (System Monitor), and PMON (Process Monitor).
81. What are database files, control files and log files. How many of these files should a database have at least? Why?
Database Files
The database files hold the actual data and are typically the largest in size. Depending on their sizes, the tables (and other objects) for all the user accounts can go in one database file—but that's not an ideal situation because it does not make the database structure very flexible for controlling access to storage for different users, putting the database on different disk drives, or backing up and restoring just part of the database.
You must have at least one database file but usually, more than one files are used. In terms of accessing and using the data in the tables and other objects, the number (or location) of the files is immaterial.
The database files are fixed in size and never grow bigger than the size at which they were created
Control Files
The control files and redo logs support the rest of the architecture. Any database must have at least one control file, although you typically have more than one to guard against loss. The control file records the name of the database, the date and time it was created, the location of the database and redo logs, and the synchronization information to ensure that all three sets of files are always in step. Every time you add a new database or redo log file to the database, the information is recorded in the control files.
Redo Logs
Any database must have at least two redo logs. These are the journals for the database; the redo logs record all changes to the user objects or system objects. If any type of failure occurs, the changes recorded in the redo logs can be used to bring the database to a consistent state without losing any committed transactions. In the case of non-data loss failure, Oracle can apply the information in the redo logs automatically without intervention from the DBA.
The redo log files are fixed in size and never grow dynamically from the size at which they were created.
82. What is ROWID?
The ROWID is a unique database-wide physical address for every row on every table. Once assigned (when the row is first inserted into the database), it never changes until the row is deleted or the table is dropped.
The ROWID consists of the following three components, the combination of which uniquely identifies the physical storage location of the row.
? Oracle database file number, which contains the block with the rows
? Oracle block address, which contains the row
? The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of retrieving rows with a particular key value. Application developers also use it in SQL statements as a quick way to access a row once they know the ROWID
83. What is Oracle Block? Can two Oracle Blocks have the same address?
Oracle "formats" the database files into a number of Oracle blocks when they are first created—making it easier for the RDBMS software to manage the files and easier to read data into the memory areas.
The block size should be a multiple of the operating system block size. Regardless of the block size, the entire block is not available for holding data; Oracle takes up some space to manage the contents of the block. This block header has a minimum size, but it can grow.
These Oracle blocks are the smallest unit of storage. Increasing the Oracle block size can improve performance, but it should be done only when the database is first created.
Each Oracle block is numbered sequentially for each database file starting at 1. Two blocks can have the same block address if they are in different database files.
84. What is database Trigger?
A database trigger is a PL/SQL block that can defined to automatically execute for insert, update, and delete statements against a table. The trigger can e defined to execute once for the entire statement or once for every row that is inserted, updated, or deleted. For any one table, there are twelve events for which you can define database triggers. A database trigger can call database procedures that are also written in PL/SQL.
85. Name two utilities that Oracle provides, which are use for backup and recovery.
Along with the RDBMS software, Oracle provides two utilities that you can use to back up and restore the database. These utilities are Export and Import.
The Export utility dumps the definitions and data for the specified part of the database to an operating system binary file. The Import utility reads the file produced by an export, recreates the definitions of objects, and inserts the data
If Export and Import are used as a means of backing up and recovering the database, all the changes made to the database cannot be recovered since the export was performed. The best you can do is recover the database to the time when the export was last performed.
86. What are stored-procedures? And what are the advantages of using them.
Stored procedures are database objects that perform a user defined operation. A stored procedure can have a set of compound SQL statements. A stored procedure executes the SQL commands and returns the result to the client. Stored procedures are used to reduce network traffic.
87. How are exceptions handled in PL/SQL? Give some of the internal exceptions' name
PL/SQL exception handling is a mechanism for dealing with run-time errors encountered during procedure execution. Use of this mechanism enables execution to continue if the error is not severe enough to cause procedure termination.
The exception handler must be defined within a subprogram specification. Errors cause the program to raise an exception with a transfer of control to the exception-handler block. After the exception handler executes, control returns to the block in which the handler was defined. If there are no more executable statements in the block, control returns to the caller.
User-Defined Exceptions
PL/SQL enables the user to define exception handlers in the declarations area of subprogram specifications. User accomplishes this by naming an exception as in the following example:
ot_failure EXCEPTION;
In this case, the exception name is ot_failure. Code associated with this handler is written in the EXCEPTION specification area as follows:
EXCEPTION
when OT_FAILURE then
out_status_code := g_out_status_code;
out_msg := g_out_msg;
The following is an example of a subprogram exception:
EXCEPTION
when NO_DATA_FOUND then
g_out_status_code := 'FAIL';
RAISE ot_failure;
Within this exception is the RAISE statement that transfers control back to the ot_failure exception handler. This technique of raising the exception is used to invoke all user-defined exceptions.
System-Defined Exceptions
Exceptions internal to PL/SQL are raised automatically upon error. NO_DATA_FOUND is a system-defined exception. Table below gives a complete list of internal exceptions.
PL/SQL internal exceptions.
Exception Name
Oracle Error
CURSOR_ALREADY_OPEN ORA-06511
DUP_VAL_ON_INDEX ORA-00001
INVALID_CURSOR ORA-01001
INVALID_NUMBER ORA-01722
LOGIN_DENIED ORA-01017
NO_DATA_FOUND ORA-01403
NOT_LOGGED_ON ORA-01012
PROGRAM_ERROR ORA-06501
STORAGE_ERROR ORA-06500
TIMEOUT_ON_RESOURCE ORA-00051
TOO_MANY_ROWS ORA-01422
TRANSACTION_BACKED_OUT ORA-00061
VALUE_ERROR ORA-06502
ZERO_DIVIDE ORA-01476
In addition to this list of exceptions, there is a catch-all exception named OTHERS that traps all errors for which specific error handling has not been established.
88. Does PL/SQL support "overloading"? Explain
The concept of overloading in PL/SQL relates to the idea that you can define procedures and functions with the same name. PL/SQL does not look only at the referenced name, however, to resolve a procedure or function call. The count and data types of formal parameters are also considered.
PL/SQL also attempts to resolve any procedure or function calls in locally defined packages before looking at globally defined packages or internal functions. To further ensure calling the proper procedure, you can use the dot notation. Prefacing a procedure or function name with the package name fully qualifies any procedure or function reference.
89. Tables derived from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes
(b) Are always in 1NF
90. Spurious tuples may occur due to
i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d) ii & iii
(a) i & iii because theta joins are joins made on keys that are not primary keys.
91. A B C is a set of attributes. The functional dependency is as follows
AB -> B
AC -> C
C -> B
a) is in 1NF
b) is in 2NF
c) is in 3NF
d) is in BCNF
(a) is in 1NF since (AC)+ = { A, B, C} hence AC is the primary key. Since C B is a FD given, where neither C is a Key nor B is a prime attribute, this it is not in 3NF. Further B is not functionally dependent on key AC thus it is not in 2NF. Thus the given FDs is in 1NF.
92. In mapping of ERD to DFD
a) entities in ERD should correspond to an existing entity/store in DFD
b) entity in DFD is converted to attributes of an entity in ERD
c) relations in ERD has 1 to 1 correspondence to processes in DFD
d) relationships in ERD has 1 to 1 correspondence to flows in DFD
(a) entities in ERD should correspond to an existing entity/store in DFD
93. A dominant entity is the entity
a) on the N side in a 1 : N relationship
b) on the 1 side in a 1 : N relationship
c) on either side in a 1 : 1 relationship
d) nothing to do with 1 : 1 or 1 : N relationship
(b) on the 1 side in a 1 : N relationship
94. Select 'NORTH', CUSTOMER From CUST_DTLS Where REGION = 'N' Order By
CUSTOMER Union Select 'EAST', CUSTOMER From CUST_DTLS Where REGION = 'E' Order By CUSTOMER
The above is
a) Not an error
b) Error - the string in single quotes 'NORTH' and 'SOUTH'
c) Error - the string should be in double quotes
d) Error - ORDER BY clause
(d) Error - the ORDER BY clause. Since ORDER BY clause cannot be used in UNIONS
95. What is Storage Manager?
It is a program module that provides the interface between the low-level data stored in database, application programs and queries submitted to the system.
96. What is Buffer Manager?
It is a program module, which is responsible for fetching data from disk storage into main memory and deciding what data to be cache in memory.
97. What is Transaction Manager?
It is a program module, which ensures that database, remains in a consistent state despite system failures and concurrent transaction execution proceeds without conflicting.
98. What is File Manager?
It is a program module, which manages the allocation of space on disk storage and data structure used to represent information stored on a disk.
99. What is Authorization and Integrity manager?
It is the program module, which tests for the satisfaction of integrity constraint and checks the authority of user to access data.
100. What are stand-alone procedures?
Procedures that are not part of a package are known as stand-alone because they independently defined. A good example of a stand-alone procedure is one written in a SQL*Forms application. These types of procedures are not available for reference from other Oracle tools. Another limitation of stand-alone procedures is that they are compiled at run time, which slows execution.
101. What are cursors give different types of cursors.
PL/SQL uses cursors for all database information accesses statements. The language supports the use two types of cursors
? Implicit
? Explicit
102. What is cold backup and hot backup (in case of Oracle)?
? Cold Backup:
It is copying the three sets of files (database files, redo logs, and control file) when the instance is shut down. This is a straight file copy, usually from the disk directly to tape. You must shut down the instance to guarantee a consistent copy.
If a cold backup is performed, the only option available in the event of data file loss is restoring all the files from the latest backup. All work performed on the database since the last backup is lost.
? Hot Backup:
Some sites (such as worldwide airline reservations systems) cannot shut down the database while making a backup copy of the files. The cold backup is not an available option.
So different means of backing up database must be used — the hot backup. Issue a SQL command to indicate to Oracle, on a tablespace-by-tablespace basis, that the files of the tablespace are to backed up. The users can continue to make full use of the files, including making changes to the data. Once the user has indicated that he/she wants to back up the tablespace files, he/she can use the operating system to copy those files to the desired backup destination.
The database must be running in ARCHIVELOG mode for the hot backup option.
If a data loss failure does occur, the lost database files can be restored using the hot backup and the online and offline redo logs created since the backup was done. The database is restored to the most consistent state without any loss of committed transactions.
103. What are Armstrong rules? How do we say that they are complete and/or sound
The well-known inference rules for FDs
? Reflexive rule :
If Y is subset or equal to X then X Y.
? Augmentation rule:
If X Y then XZ YZ.
? Transitive rule:
If {X Y, Y Z} then X Z.
? Decomposition rule :
If X YZ then X Y.
? Union or Additive rule:
If {X Y, X Z} then X YZ.
? Pseudo Transitive rule :
If {X Y, WY Z} then WX Z.
Of these the first three are known as Amstrong Rules. They are sound because it is enough if a set of FDs satisfy these three. They are called complete because using these three rules we can generate the rest all inference rules.
104. How can you find the minimal key of relational schema?
Minimal key is one which can identify each tuple of the given relation schema uniquely. For finding the minimal key it is required to find the closure that is the set of all attributes that are dependent on any given set of attributes under the given set of functional dependency.
Algo. I Determining X+, closure for X, given set of FDs F
1. Set X+ = X
2. Set Old X+ = X+
3. For each FD Y Z in F and if Y belongs to X+ then add Z to X+
4. Repeat steps 2 and 3 until Old X+ = X+
Algo.II Determining minimal K for relation schema R, given set of FDs F
1. Set K to R that is make K a set of all attributes in R
2. For each attribute A in K
a. Compute (K – A)+ with respect to F
b. If (K – A)+ = R then set K = (K – A)+
105. What do you understand by dependency preservation?
Given a relation R and a set of FDs F, dependency preservation states that the closure of the union of the projection of F on each decomposed relation Ri is equal to the closure of F. i.e.,
((?R1(F)) U … U (?Rn(F)))+ = F+
if decomposition is not dependency preserving, then some dependency is lost in the decomposition.
106. What is meant by Proactive, Retroactive and Simultaneous Update.
Proactive Update:
The updates that are applied to database before it becomes effective in real world .
Retroactive Update:
The updates that are applied to database after it becomes effective in real world .
Simulatneous Update:
The updates that are applied to database at the same time when it becomes effective in real world .
107. What are the different types of JOIN operations?
Equi Join: This is the most common type of join which involves only equality comparisions. The disadvantage in this type of join is that there
Touch Screen
How Does Touch screen Function
A basic touch screen has three main components a touch sensor, a controller, and a software driver. The touch screen is an input device it needs to be combined with a display and a PC or other device to make a complete touch input system.
1. Touch sensor:
A touch sensor is a clear glass panel with a touch responsive surface. The touch sensor/panel is placed over a display screen so that the responsive area of the panel covers the viewable area of the video screen.
There are several different touch sensor technologies on the market today, each using a different method to detect touch input.
The sensor generally has an electrical current or signal going through it and touching the screen causes a voltage or signal change. This voltage change is used to determine the location of the touch to the screen.
2. Controller:
The controller is a small PC card that connects between the touch sensor and the PC. It takes information from the touch sensor and translates it into information that PC can understand. The controller is usually installed inside the monitor for integrated monitors or it is housed in a plastic case for external add-ons / overlays.
The controller determines what type of interface/connection you will need on the PC .Integrated touch monitors will have an extra cable connection on the back for the touch screen. Controllers are available that can connect to a serial/COM port (PC) or to a USB port (PC or Macintosh).Specialized controllers are also available that work with DVD players and other devices.
3. Software Driver:
The driver is a software update for the PC system that allows the touch screen and computer to work together. It tells the computer’s operating system how to interpret the touch event information that is sent from the controller. Most touch screen drivers today are a mouse-emulation toe driver.
This makes touching the screen the same as clicking your mouse at the same location on the screen. This allows the touch screen to work with existing software and allows new applications to be developed without the need for touch screen specific programming. Some equipment such as thin client terminals, DVD players, and specialized computer systems either do not use software drivers or they have their own built-in touch screen driver.
All of the touch screens that a re on offer basically work like mouse. Once the software driver for the touch screen is installed, the touch screen emulates mouse functions. Touching the screen is basically the same as clicking your mouse at the same point at the screen.
When you touch the touch screen, the mouse cursor will move to that point and make a mouse click. You can tap the screen twice to perform a double-click, and can also drag your finger across the touch screen to perform drag-and-drops. The touch screens normally emulate left mouse clicks. Through software, you can also switch the touch screen to perform right mouse clicks instead.
Bluetooth
How does data get transferred through Bluetooth?
Bluetooth is a short range wireless communications technology intended to replace the cables connecting portable and/or fixed devices while maintaining high levels of security. The key features’ of Bluetooth technology are robustness, low power, and low cost.
Bluetooth enabled electronic devices transfer data; connect and communicate wirelessly through short –range, adhoc networks known as piconets.
Each device can simultaneously communicate with up to seven other device within a single piconet.
Each device can also belong to several piconets simultaneously. Piconets are established dynamically and automatically as Bluetooth enabled devices enter and leave radio proximity.
A fundamental Bluetooth wireless technology strength is the ability to simultaneously handle both data and voice transmissions. This enables users to enjoy variety of innovative solutions such as handles – free headset fro voice calls, printing and fax capabilities and, most importantly, synchronizing a PDA and /or laptop with a Disto A6 or Disto plus laser measuring device.
Range: The operating range depends on the device class;
Class 3 – have a range of up to 1 meter or 3 feet
Class 2 – most commonly found in mobile devices – have a range of 0 meters or 30 feet
Classes 1 – used primarily in industrial use cases – have a range of 100 meters or 300 feet. The Disto A6 and Disto plus are class 2 devices.
Power: The most commonly used radio is class 2 and uses 2.5mW of power. Bluetooth technology is designed to have very low power consumption.
Godwin Dinesh .MCA
System Engineer
Domain name System (DNS)
DNS stands fro Domain name system resolves hostname from IP addresses a look up on the DNS table will be performed which will retrieve the IP address otherwise it would have very for human beings to remember IP addresses but hostname are easy to remember.
www.google.com (its is a domain name) is much easier to remember than an IP address 64. 233.167.99, so all of them kindly remember the different in IP and domain name.
What Is DHCP?
DHCP stands for dynamic host configuration protocol .DHCP server dynamically assign Ip addresses (with the help of IP address one computer communicate with other computer) to clients (hosts) rather than network administration provide IP address to client statically . So it makes the administration purposes simpler.
Linux BASH command line
An A-Z Index of the Linux BASH command line
alias Create an alias
apropos Search Help manual pages (man -k)
awk Find and Replace text, database sort/validate/index
break Exit from a loop
builtin Run a shell builtin
bzip2 Compress or decompress named file(s)
cal Display a calendar
case Conditionally perform a command
cat Display the contents of a file
cd Change Directory
cfdisk Partition table manipulator for Linux
chgrp Change group ownership
chmod Change access permissions
chown Change file owner and group
chroot Run a command with a different root directory
cksum Print CRC checksum and byte counts
clear Clear terminal screen
cmp Compare two files
comm Compare two sorted files line by line
command Run a command - ignoring shell functions
continue Resume the next iteration of a loop
cp Copy one or more files to another location
cron Daemon to execute scheduled commands
crontab Shedule a command to run at a later time
csplit Split a file into context-determined pieces
cut Divide a file into several parts
date Display or change the date & time
dc Desk Calculator
dd Data Dump - Convert and copy a file
ddrescue Data recovery tool
declare Declare variables and give them attributes
df Display free disk space
diff Display the differences between two files
diff3 Show differences among three files
dig DNS lookup
dir Briefly list directory contents
dircolors Colour setup for `ls'
dirname Convert a full pathname to just a path
dirs Display list of remembered directories
du Estimate file space usage
echo Display message on screen
egrep Search file(s) for lines that match an extended expression
eject Eject removable media
enable Enable and disable builtin shell commands
env Environment variables
ethtool Ethernet card settings
eval Evaluate several commands/arguments
exec Execute a command
exit Exit the shell
expand Convert tabs to spaces
export Set an environment variable
expr Evaluate expressions
false Do nothing, unsuccessfully
fdformat Low-level format a floppy disk
fdisk Partition table manipulator for Linux
fgrep Search file(s) for lines that match a fixed string
file Determine file type
find Search for files that meet a desired criteria
fmt Reformat paragraph text
fold Wrap text to fit a specified width.
for Expand words, and execute commands
format Format disks or tapes
free Display memory usage
fsck File system consistency check and repair
ftp File Transfer Protocol
function Define Function Macros
gawk Find and Replace text within file(s)
getopts Parse positional parameters
grep Search file(s) for lines that match a given pattern
groups Print group names a user is in
gzip Compress or decompress named file(s)
hash Remember the full pathname of a name argument
head Output the first part of file(s)
history Command History
hostname Print or set system name
id Print user and group id's
if Conditionally perform a command
ifconfig Configure a network interface
import Capture an X server screen and save the image to file
install Copy files and set attributes
join Join lines on a common field
kill Stop a process from running
less Display output one screen at a time
let Perform arithmetic on shell variables
ln Make links between files
local Create variables
locate Find files
logname Print current login name
logout Exit a login shell
look Display lines beginning with a given string
lpc Line printer control program
lpr Off line print
lprint Print a file
lprintd Abort a print job
lprintq List the print queue
lprm Remove jobs from the print queue
ls List information about file(s)
lsof List open files
make Recompile a group of programs
man Help manual
mkdir Create new folder(s)
mkfifo Make FIFOs (named pipes)
mkisofs Create an hybrid ISO9660/JOLIET/HFS filesystem
mknod Make block or character special files
more Display output one screen at a time
mount Mount a file system
mtools Manipulate MS-DOS files
mv Move or rename files or directories
netstat Networking information
nice Set the priority of a command or job
nl Number lines and write files
nohup Run a command immune to hangups
nslookup Query Internet name servers interactively
passwd Modify a user password
paste Merge lines of files
pathchk Check file name portability
ping Test a network connection
popd Restore the previous value of the current directory
pr Prepare files for printing
printcap Printer capability database
printenv Print environment variables
printf Format and print data
ps Process status
pushd Save and then change the current directory
pwd Print Working Directory
quota Display disk usage and limits
quotacheck Scan a file system for disk usage
quotactl Set disk quotas
ram ram disk device
rcp Copy files between two machines.
read read a line from standard input
readonly Mark variables/functions as readonly
remsync Synchronize remote files via email
return Exit a shell function
rm Remove files
rmdir Remove folder(s)
rsync Remote file copy (Synchronize file trees)
screen Terminal window manager
scp Secure copy (remote file copy)
sdiff Merge two files interactively
sed Stream Editor
select Accept keyboard input
seq Print numeric sequences
set Manipulate shell variables and functions
sftp Secure File Transfer Program
shift Shift positional parameters
shopt Shell Options
shutdown Shutdown or restart linux
sleep Delay for a specified time
sort Sort text files
source Run commands from a file `.'
split Split a file into fixed-size pieces
ssh Secure Shell client (remote login program)
strace Trace system calls and signals
su Substitute user identity
sudo Execute a command as another user
sum Print a checksum for a file
symlink Make a new name for a file
sync Synchronize data on disk with memory
tail Output the last part of files
tar Tape ARchiver
tee Redirect output to multiple files
test Evaluate a conditional expression
time Measure Program running time
times User and system times
touch Change file timestamps
top List processes running on the system
traceroute Trace Route to Host
trap Run a command when a signal is set(bourne)
tr Translate, squeeze, and/or delete characters
true Do nothing, successfully
tsort Topological sort
tty Print filename of terminal on stdin
type Describe a command
ulimit Limit user resources
umask Users file creation mask
umount Unmount a device
unalias Remove an alias
uname Print system information
unexpand Convert spaces to tabs
uniq Uniquify files
units Convert units from one scale to another
unset Remove variable or function names
unshar Unpack shell archive scripts
until Execute commands (until error)
useradd Create new user account
usermod Modify user account
users List users currently logged in
uuencode Encode a binary file
uudecode Decode a file created by uuencod
v Verbosely list directory contents (`ls -l -b')
vdir Verbosely list directory contents (`ls -l -b')
vi Text Editor
watch Execute/display a program periodically
wc Print byte, word, and line counts
whereis Report all known instances of a command
which Locate a program file in the user's path.
while Execute commands
who Print all usernames currently logged in
whoami Print the current user id and name (`id -un')
Wget Retrieve web pages or files via HTTP, HTTPS or FTP
xargs Execute utility, passing constructed argument list(s)
yes Print a string until interrupted
. Run a command script in the current shell
### Comment / Remark
Godwin Dinesh.MCA
BANDWIDTH
The Internet consists of tens of millions of computers throughout the world, all connected by cables. Your ISP (Internet Service Provider) is a company that has a very high-speed (and very expensive) connection to the Internet. Your ISP makes its money by renting little "chunks" of that high-speed connection to consumers who don't want to spend thousands of dollars each month for an Internet connection.
If you've ever wondered why it takes so long to download certain Web pages or other files to your computer, it's all determined by the bandwidth of the connection between your computer and your Internet Service Provider. Which is to say, it all hinges on that wire connecting your computer to your modem to your ISP. That connection is often referred to as the last mile, as illustrated .
When you open a Web page or download a file, it travels very quickly from wherever it happens to be on the planet Earth to your ISP. It then travels from the ISP's computers to your computer. The actual speed at which information moves through a wire is always the same, no matter what your bandwidth might be. That is, electrons run through the wires at close to the speed of light, about 670,000,000 MPH (six hundred and seventy million miles per hour). At that speed, you could circle the globe 7 times a second, or 420 times a minute. So why does it take so long for every Web page you visit to appear on your screen? That's where bandwidth comes in.
Bandwidth is easy to understand if we use an analogy from everyday life. Imagine that instead of trying to get a Web page or file from your ISP through wires, you're trying to drain a swimming pool. The water in the pool is the Web page or file. If you stick a skinny hose to drain the water from the pool, as in , it will take quite a while to drain all the water.
Now let's say that instead of sticking a skinny pipe onto the pool we put on a big fat pipe, as in Figure 3. I don’t think it takes a physics major to realize that it’s going to take less time to drain the pool in Figure 3 than it is to drain the pool .
We can say that the fat drain pipe in is "wider" than the pipe in Figure 2. Or, we could take it another step and say that the fat pipe in Figure 3 has more bandwidth than the skinny pipe. And ultimately, that's what bandwidth is all about. How much stuff can pass through the pipe (or wire) at a time.
Dial-up Internet accounts, which use a standard telephone line to connect to an ISP, have a very narrow bandwidth (about 50 Kbps or 50,000 bits per second). Thus, things are slow in the sense that it takes a long time to download things.
A broadband Internet account can move data at anywhere from 128 Kbps to 2,000 Kbps or more. That's like the fat drain pipe. It takes a lot less time to get a Web page or file from your ISP's computer to your computer using the broadband account than it does the dial-up account.
So the whole bandwidth thing (like many things "computerish") boils down to a time vs. money decision. You can either spend the extra amount per month for a faster connection, and spend less time waiting for things to download. Or, you spend less money on a dial-up account, but spend more time waiting for things to appear on your screen.
Godwin Dinesh MCA
SQL - Structure Query language
What is SQL?
· SQL stands for Structured Query Language
· SQL allows you to access a database
· SQL is an ANSI standard computer language
· SQL can execute queries against a database
· SQL can retrieve data from a database
· SQL can insert new records in a database
· SQL can delete records from a database
· SQL can update records in a database
· SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
SELECT LastName FROM Persons
Gives a result set like this:
LastName
Hansen
Svendson
Pettersen
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
· SELECT - extracts data from a database table
· UPDATE - updates data in a database table
· DELETE - deletes data from a database table
· INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
· CREATE TABLE - creates a new database table
· ALTER TABLE - alters (changes) a database table
· DROP TABLE - deletes a database table
· CREATE INDEX - creates an index (search key)
· DROP INDEX - deletes an index
The SQL SELECT Statement
The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set).
Syntax
SELECT column_name(s) FROM table_name
Note: SQL statements are not case sensitive. SELECT is the same as select.
SQL SELECT Example
To select the content of columns named "LastName" and "FirstName", from the database table called "Persons", use a SELECT statement like this:
SELECT LastName,FirstName FROM Persons
The database table "Persons":
LastName | FirstName | Address | City |
|
Hansen | Ola | Timoteivn 10 | Sandnes |
|
Svendson | Tove | Borgvn 23 | Sandnes |
|
Pettersen | Kari | Storgt 20 | Stavanger |
|
The result
LastName FirstName
Hansen Ola
Svendson Tove
Pettersen Kari
Select All Columns
To select all columns from the "Persons" table, use a * symbol instead of column names, like this:
SELECT * FROM Persons
Result
LastName FirstName Address City
Hansen Ola Timoteivn 10 Sandnes
Svendson Tove Borgvn 23 Sandnes
Pettersen Kari Storgt 20 Stavanger
The Result Set
The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial .
Semicolon after SQL Statements?
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
The SELECT DISTINCT Statement
The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:
Syntax
SELECT DISTINCT column_name(s) FROM table_name
Using the DISTINCT keyword
To select ALL values from the column named "Company" we use a SELECT statement like this:
SELECT Company FROM Orders
"Orders" table
Company OrderNumber
Sega 3412
W3Schools 2312
Trio 4678
W3Schools 6798
Result
Company
Sega
W3Schools
Trio
W3Schools
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
SELECT DISTINCT Company FROM Orders
Result:
Company
Sega
W3Schools
Trio
Now "W3Schools" is listed only once in the result-set.
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
SELECT column FROM table WHERE column operator value
With the WHERE clause, the following operators can be used:
Operator Description
= Equal
<> Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
Note: In some versions of SQL the <> operator may be written as !=
Using the WHERE Clause
To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement:
SELECT * FROM Persons WHERE City='Sandnes'
"Persons" table
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Pettersen Kari Storgt 20 Stavanger 1960
Result
LastName FirstName Address City Year
Hansen Ola Timoteivn 10 Sandnes 1951
Svendson Tove Borgvn 23 Sandnes 1978
Svendson Stale Kaivn 18 Sandnes 1980
Using Quotes
Note that we have used single quotes around the conditional values in the examples.
SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes.
For text values:
This is correct: SELECT * FROM Persons WHERE FirstName='Tove' This is wrong: SELECT * FROM Persons WHERE FirstName=Tove
For numeric values:
This is correct: SELECT * FROM Persons WHERE Year>1965 This is wrong: SELECT * FROM Persons WHERE Year>'1965'
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
SELECT column FROM table WHERE column LIKE pattern
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
SELECT * FROM Persons WHERE FirstName LIKE 'O%'
The following SQL statement will return persons with first names that end with an 'a':
SELECT * FROM Persons WHERE FirstName LIKE '%a'
The following SQL statement will return persons with first names that contain the pattern 'la':
SELECT * FROM Persons WHERE FirstName LIKE '%la%'
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
INSERT INTO table_name VALUES (value1, value2,....)
You can also specify the columns for which you want to insert data:
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)
Insert a New Row
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
And this SQL statement:
INSERT INTO Persons VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Insert Data in Specified Columns
This "Persons" table:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
And This SQL statement:
INSERT INTO Persons (LastName, Address) VALUES ('Rasmussen', 'Storgt 67')
Will give this result:
LastName FirstName Address City
Pettersen Kari Storgt 20 Stavanger
Hetland Camilla Hagabakka 24 Sandnes
Rasmussen Storgt 67
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
UPDATE table_name SET column_name = new_value WHERE column_name = some_value
Person:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Storgt 67
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
UPDATE Person SET FirstName = 'Nina' WHERE LastName = 'Rasmussen'
Result:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Storgt 67
Update several Columns in a Row
We want to change the address and add the name of the city:
UPDATE Person SET Address = 'Stien 12', City = 'Stavanger' WHERE LastName = 'Rasmussen'
Result:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Stien 12 Stavanger
The DELETE Statement
The DELETE statement is used to delete rows in a table.
Syntax
DELETE FROM table_name WHERE column_name = some_value
Person:
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Rasmussen Nina Stien 12 Stavanger
Delete a Row
"Nina Rasmussen" is going to be deleted:
DELETE FROM Person WHERE LastName = 'Rasmussen'
Result
LastName FirstName Address City
Nilsen Fred Kirkegt 56 Stavanger
Delete All Rows
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_name or DELETE * FROM table_name
Test your SQL Skills
On this page you can test your SQL skills.
We will use the Customers table in the Northwind database:
CompanyName | ContactName | Address City |
|
Alfreds Futterkiste | Maria Anders | Obere Str. 57 Berlin |
|
Berglunds snabbköp | Christina Berglund | sssssssssssBerguvsvägen 8 Luleå |
Centro comercial Moctezuma | Francisco Chang | Sierras de Granada 9993 México D.F. |
Ernst Handel | Roland Mendel | Kirchgasse 6 Graz |
FISSA Fabrica Inter. Salchichas S.A. Diego Roel | C/ Moralzarzal, 86 Madrid |
|
GalerÃa del gastrónomo | Eduardo Saavedra | Rambla de Cataluña, 23 Barcelona |
Island Trading | Helen Bennett | Garden House Crowther Way Cowes |
Königlich Essen | Philip Cramer | Maubelstr. 90 Brandenburg |
Laughing Bacchus Wine Cellars | Yoshi Tannamuri | 1900 Oak St. Vancouver |
Magazzini Alimentari Riuniti | Giovanni Rovelli | Via Ludovico il Moro 22 Bergamo |
North/South | Simon Crowther | South House 300 Queensbridge |
Paris spécialités | Marie Bertrand | 265, boulevard Charonne |
Rattlesnake Canyon Grocery | Paula Wilson | 2817 Milton Dr. Albuquerque |
Simons bistro | Jytte Petersen | Vinbæltet 34 København |
The Big Cheese | Liz Nixon | 89 Jefferson Way Suite 2 |
Vaffeljernet | Palle Ibsen | Smagsløget 45 Århus |
Wolski Zajazd | Zbyszek Piestrzeniewicz ul.Filtrowa 68 Warszawa |
|
To preserve space, the table above is a subset of the Customers table used in the example below.
Try it Yourself
To see how SQL works, you can copy the SQL statements below and paste them into the textarea, or you can make your own SQL statements.
SELECT * FROM customers
SELECT CompanyName, ContactName FROM customers
SELECT * FROM customers WHERE companyname LIKE 'a%'
SELECT CompanyName, ContactName FROM customers WHERE CompanyName > 'g' AND ContactName > 'g'
The ORDER BY keyword is used to sort the result.
Sort the Rows
The ORDER BY clause is used to sort the rows.
Orders:
Company | OrderNumber |
Sega | 3412 |
ABC Shop | 5678 |
W3Schools | 2312 |
W3Schools | 6798 |
Example
To display the company names in alphabetical order:
SELECT Company, OrderNumber FROM Orders ORDER BY Company
Result:
Company | OrderNumber |
ABC Shop | 5678 |
Sega | 3412 |
W3Schools | 6798 |
W3Schools | 2312 |
Example
To display the company names in alphabetical order AND the OrderNumber in numerical order:
SELECT Company, OrderNumber FROM Orders ORDER BY Company, OrderNumber
Result:
ABC Shop | 5678 |
Sega Company | OrderNumber |
| 3412 |
W3Schools | 2312 |
W3Schools | 6798 |
Example
To display the company names in reverse alphabetical order:
SELECT Company, OrderNumber FROM Orders ORDER BY Company DESC
Result:
Company | OrderNumber |
W3Schools | 6798 |
W3Schools | 2312 |
Sega | 3412 |
ABC Shop | 5678 |
Example
To display the company names in reverse alphabetical order AND the OrderNumber in numerical order:
SELECT Company, OrderNumber FROM Orders ORDER BY Company DESC, OrderNumber ASC
Result:
Company | OrderNumber |
W3Schools | 2312 |
W3Schools | 6798 |
Sega | 3412 |
ABC Shop | 5678 |
Notice that there are two equal company names (W3Schools) in the result above. The only time you will see the second column in ASC order would be when there are duplicated values in the first sort column, or a handful of nulls.
AND & OR
AND and OR join two or more conditions in a WHERE clause.
The AND operator displays a row if ALL conditions listed are true. The OR operator displays a row if ANY of the conditions listed are true.
Original Table (used in the examples)
LastName FirstName Address City |
Hansen Ola Timoteivn 10 Sandnes |
Svendson Tove Borgvn 23 Sandnes |
Svendson Stephen Kaivn 18 Sandnes |
|
Example
Use AND to display each person with the first name equal to "Tove", and the last name equal to "Svendson":
SELECT * FROM Persons WHERE FirstName='Tove' AND LastName='Svendson'
Result:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Example
Use OR to display each person with the first name equal to "Tove", or the last name equal to "Svendson":
SELECT * FROM Persons WHERE firstname='Tove' OR lastname='Svendson'
Result:
LastName | FirstName | Address | City |
Svendson | Tove | Borgvn 23 | Sandnes |
Svendson | Stephen | Kaivn 18 | Sandnes |
Example
You can also combine AND and OR (use parentheses to form complex expressions):
SELECT * FROM Persons WHERE (FirstName='Tove' OR FirstName='Stephen') AND LastName='Svendson'
Result:
LastName FirstName Address City |
Svendson Tove Borgvn 23 Sandnes |
Svendson Stephen Kaivn 18 Sandnes |
IN
The IN operator may be used if you know the exact value you want to return for at least one of the columns.
SELECT column_name FROM table_name WHERE column_name IN (value1,value2,..)
Original Table (used in the examples)
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Nordmann | Anna | Neset 18 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
Svendson | Tove | Borgvn 23 | Sandnes |
Example 1
To display the persons with LastName equal to "Hansen" or "Pettersen", use the following SQL:
SELECT * FROM Persons WHERE LastName IN ('Hansen','Pettersen')
Result: |
LastName FirstName Address City |
Hansen Ola Timoteivn 10 Sandnes |
Pettersen Kari Storgt 20 Stavanger |
BETWEEN ... AND
The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.
SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND value2
Original Table (used in the examples)
LastName FirstName Address City |
Hansen Ola Timoteivn 10 Sandnes |
Nordmann Anna Neset 18 Sandnes |
Pettersen Kari Storgt 20 Stavanger |
Svendson Tove Borgvn 23 Sandnes |
|
Example 1
To display the persons alphabetically between (and including) "Hansen" and exclusive "Pettersen", use the following SQL:
SELECT * FROM Persons WHERE LastName BETWEEN 'Hansen' AND 'Pettersen'
Result:
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Nordmann | Anna | Neset 18 | Sandnes |
IMPORTANT! The BETWEEN...AND operator is treated differently in different databases. With some databases a person with the LastName of "Hansen" or "Pettersen" will not be listed (BETWEEN..AND only selects fields that are between and excluding the test values). With some databases a person with the last name of "Hansen" or "Pettersen" will be listed (BETWEEN..AND selects fields that are between and including the test values). With other databases a person with the last name of "Hansen" will be listed, but "Pettersen" will not be listed (BETWEEN..AND selects fields between the test values, including the first test value and excluding the last test value). Therefore: Check how your database treats the BETWEEN....AND operator!
Example 2
To display the persons outside the range used in the previous example, use the NOT operator:
SELECT * FROM Persons WHERE LastName NOT BETWEEN 'Hansen' AND 'Pettersen'
Result:
LastName | FirstName | Address | City |
Pettersen | Kari | Storgt 20 | Stavanger |
Svendson | Tove | Borgvn 23 | Sandnes |
Column Name Alias
The syntax is:
SELECT column AS column_alias FROM table
Table Name Alias
The syntax is:
SELECT column FROM table AS table_alias
Example: Using a Column Alias
This table (Persons):
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Svendson | Tove | Borgvn 23 | Sandnes |
Pettersen | Kari | Storgt 20 | Stava |
nger
And this SQL:
SELECT LastName AS Family, FirstName AS Name FROM Persons
Returns this result:
Family | Name |
Hansen | Ola |
Svendson | Tove |
Pettersen | Kari |
Example: Using a Table Alias
This table (Persons):
LastName | FirstName | Address | City |
Hansen | Ola | Timoteivn 10 | Sandnes |
Svendson | Tove | Borgvn 23 | Sandnes |
Pettersen | Kari | Storgt 20 | Stavanger |
And this SQL:
SELECT LastName, FirstName FROM Persons AS Employees
Returns this result:
Table Employees:
LastName | FirstName |
Hansen | Ola |
Svendson | Tove |
Pettersen | Kari |
Joins and Keys
Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:
· The "Employee_ID" column is the primary key of the "Employees" table
· The "Prod_ID" column is the primary key of the "Orders" table
· The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names
Employees:
Employee_ID | NAME |
01 | Hansen, Ola |
02 | Svendson, Tove |
03 | Svendson, Stephen |
04 | Pettersen, Kari |
Orders:
Prod_ID Product Employee_ID |
234 Printer 01 |
657 Table 03 |
865 Chair 03 |
|
Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.Product FROM Employees, Orders WHERE Employees.Employee_ID=Orders.Employee_ID
Result
Name Product
Hansen, Ola | Printer |
Svendson, Stephen | Table |
Svendson, Stephen | Chair |
Example
Who ordered a printer?
SELECT Employees.Name FROM Employees, Orders WHERE Employees.Employee_ID=Orders.Employee_ID AND Orders.Product='Printer'
Result
Name |
Hansen, Ola |
Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax
SELECT field1, field2, field3 FROM first_table INNER JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield
Who has ordered a product, and what did they order?
SELECT Employees.Name, Orders.Product FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID
The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result
Name Product |
Hansen, Ola Printer |
Svendson, Stephen Table |
Svendson, Stephen Chair |
|
Example LEFT JOIN
Syntax
SELECT field1, field2, field3 FROM first_table LEFT JOIN second_table ON first_table.keyfield = second_table.foreign_keyfields
List all employees, and their orders - if any.
SELECT Employees.Name, Orders.Product FROM Employees LEFT JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID
The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result
Name Product |
Hansen, Ola Printer |
Svendson, Tove |
Svendson, Stephen Table |
Svendson, Stephen Chair |
Pettersen, Kari |
Example RIGHT JOIN
Syntax
SELECT field1, field2, field3 FROM first_table RIGHT JOIN second_table ON first_table.keyfield = second_table.foreign_keyfield
List all orders, and who has ordered - if any.
SELECT Employees.Name, Orders.Product FROM Employees RIGHT JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result
Name | Product |
Hansen, Ola | Printer |
Svendson, Stephen | Table |
Svendson, Stephen | Chair |
Example
Who ordered a printer?
SELECT Employees.Name FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID WHERE Orders.Product = 'Printer'
Result
Name
Hansen, Ola
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
Note: With UNION, only distinct values are selected.
SQL Statement 1 UNION SQL Statement 2
Employees_Norway: |
E_ID E_Name |
01 Hansen, Ola |
02 Svendson, Tove |
03 Svendson, Stephen |
04 Pettersen, Kari |
Employees_USA: |
E_ID E_Name |
01 Turner, Sally |
02 Kent, Clark |
03 Svendson, Stephen |
04 Scott, Stephen |
|
Using the UNION Command
Example
List all different employee names in Norway and USA:
SELECT E_Name FROM Employees_Norway UNION SELECT E_Name FROM Employees_USA
Result
E_Name |
Hansen, Ola |
Svendson, Tove |
Svendson, Stephen |
Pettersen, Kari |
Turner, Sally |
Kent, Clark |
Scott, Stephen |
Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
SQL Statement 1 UNION ALL SQL Statement 2
Using the UNION ALL Command
Example
List all employees in Norway and USA:
SELECT E_Name FROM Employees_Norway UNION ALL SELECT E_Name FROM Employees_USA
Result
E_Name |
Hansen, Ola |
Svendson, Tove |
Svendson, Stephen |
Pettersen, Kari |
Turner, Sally |
Kent, Clark |
Svendson, Stephen |
Scott, Stephen |
Create a Database
To create a database:
CREATE DATABASE database_name
Create a Table
To create a table in a database:
CREATE TABLE table_name ( column_name1 data_type, column_name2 data_type, ....... )
Example
This example demonstrates how you can create a table named "Person", with four columns. The column names will be "LastName", "FirstName", "Address", and "Age":
CREATE TABLE Person ( LastName varchar, FirstName varchar, Address varchar, Age int )
This example demonstrates how you can specify a maximum length for some columns:
CREATE TABLE Person ( LastName varchar(30), FirstName varchar, Address varchar, Age int(3) )
The data type specifies what type of data the column can hold. The table below contains the most common data types in SQL:
Data Type Description |
integer(size) int(size) smallint(size) Hold integers only. The maximum number |
tinyint(size) of digits are specified in parenthesis. |
decimal(size,d) numeric(size,d) Hold numbers with fractions. The maximum number of digits are specified in "size". The maximum number of digits to the right of the decimal is specified in "d". |
char(size) Holds a fixed length string (can contain letters, numbers, and special characters). The fixed size is specified in parenthesis. |
varchar(size) Holds a variable length string (can contain letters, numbers, and special characters). The maximum size is specified in parenthesis. |
date(yyyymmdd) Holds a date |
Create Index
Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries.
Note: Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update. So, it is a good idea to create indexes only on columns that are often used for a search.
A Unique Index
Creates a unique index on a table. A unique index means that two rows cannot have the same index value.
CREATE UNIQUE INDEX index_name ON table_name (column_name)
The "column_name" specifies the column you want indexed.
A Simple Index
Creates a simple index on a table. When the UNIQUE keyword is omitted, duplicate values are allowed.
CREATE INDEX index_name ON table_name (column_name)
The "column_name" specifies the column you want indexed.
Example
This example creates a simple index, named "PersonIndex", on the LastName field of the Person table:
CREATE INDEX PersonIndex ON Person (LastName)
If you want to index the values in a column in descending order, you can add the reserved word DESC after the column name:
CREATE INDEX PersonIndex ON Person (LastName DESC)
If you want to index more than one column you can list the column names within the parentheses, separated by commas:
CREATE INDEX PersonIndex ON Person (LastName, FirstName)
Drop Index
You can delete an existing index in a table with the DROP INDEX statement.
Syntax for Microsoft SQLJet (and Microsoft Access):
DROP INDEX index_name ON table_name
Syntax for MS SQL Server:
DROP INDEX table_name.index_name
Syntax for IBM DB2 and Oracle:
DROP INDEX index_name
Syntax for MySQL:
ALTER TABLE table_name DROP INDEX index_name
Delete a Table or Database
To delete a table (the table structure, attributes, and indexes will also be deleted):
DROP TABLE table_name
To delete a database:
DROP DATABASE database_name
Truncate a Table
What if we only want to get rid of the data inside a table, and not the table itself? Use the TRUNCATE TABLE command (deletes only the data inside the table):
TRUNCATE TABLE table_name
ALTER TABLE
The ALTER TABLE statement is used to add or drop columns in an existing table.
ALTER TABLE table_name ADD column_name datatype ALTER TABLE table_name DROP COLUMN column_name
Note: Some database systems don't allow the dropping of a column in a database table (DROP COLUMN column_name).
Person:
LastName FirstName Address |
Pettersen Kari Storgt 20 |
Example
To add a column named "City" in the "Person" table:
ALTER TABLE Person ADD City varchar(30)
Result:
LastName | FirstName | Address | City |
Pettersen | Kari | Storgt 20 |
|
Example
To drop the "Address" column in the "Person" table:
ALTER TABLE Person DROP COLUMN Address
Result:
LastName FirstName City |
Pettersen Kari |
|
SQL has a lot of built-in functions for counting and calculations.
Function Syntax
The syntax for built-in SQL functions is:
SELECT function(column) FROM table
Types of Functions
There are several basic types and categories of functions in SQL. The basic types of functions are:
· Aggregate Functions
· Scalar functions
Aggregate functions
Aggregate functions operate against a collection of values, but return a single value.
Note: If used among many other expressions in the item list of a SELECT statement, the SELECT must have a GROUP BY clause!!
"Persons" table (used in most examples)
Name Age |
Hansen, Ola 34 |
Svendson, Tove 45 |
Pettersen, Kari 19 |
|
Aggregate functions in MS Access
Function Description
AVG(column) Returns the average value of a column |
COUNT(column) Returns the number of rows (without a NULL value) of a column |
COUNT(*) Returns the number of selected rows |
FIRST(column) Returns the value of the first record in a specified field |
LAST(column) Returns the value of the last record in a specified field |
MAX(column) Returns the highest value of a column |
MIN(column) Returns the lowest value of a column |
STDEV(column) |
STDEVP(column) |
SUM(column) Returns the total sum of a column |
VAR(column) |
VARP(column) |
Aggregate functions in SQL Server
Function Description |
AVG(column) Returns the average value of a column |
BINARY_CHECKSUM |
CHECKSUM |
CHECKSUM_AGG |
COUNT(column) Returns the number of rows (without a NULL value) of a column |
COUNT(*) Returns the number of selected rows |
COUNT(DISTINCT column) Returns the number of distinct results |
FIRST(column) Returns the value of the first record in a specified field |
(not supported in SQLServer2K) |
LAST(column) Returns the value of the last record in a specified field |
(not supported in SQLServer2K) |
MAX(column) Returns the highest value of a column |
MIN(column) Returns the lowest value of a column |
STDEV(column) |
STDEVP(column) |
SUM(column) Returns the total sum of a column |
VAR(column) |
VARP(column |
)
Scalar functions
Scalar functions operate against a single value, and return a single value based on the input value.
Useful Scalar Functions in MS Access
Function | Description |
|
UCASE(c) | Converts a field to upper case |
|
LCASE(c) | Converts a field to lower case |
MID(c,start[,end]) | Extract characters from a text field |
LEN(c) | Returns the length of a text field |
INSTR(c,char) | Returns the numeric position of a named character within a text field |
LEFT(c,number_of_char) | Return the left part of a text field requested |
RIGHT(c,number_of_char) | Return the right part of a text field requested |
ROUND(c,decimals) | Rounds a numeric field to the number of decimals specified |
MOD(x,y) | Returns the remainder of a division operation |
NOW() | Returns the current system date |
FORMAT(c,format) | Changes the way a field is displayed |
DATEDIFF(d,date1,date2) | Used to perform date calculations |
|
|
Aggregate functions (like SUM) often need an added GROUP BY functionality.
GROUP BY...
GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.
The syntax for the GROUP BY function is:
SELECT column,SUM(column) FROM table GROUP BY column
GROUP BY Example
This "Sales" Table:
Company Amount
W3Schools | 5500 |
IBM | 4500 |
W3Schools | 7100 |
And This SQL:
SELECT Company, SUM(Amount) FROM Sales
Returns this result:
Company SUM(Amount)
W3Schools | 17100 |
IBM | 17100 |
W3Schools | 17100 |
The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:
SELECT Company,SUM(Amount) FROM Sales GROUP BY Company
Returns this result:
Company SUM(Amount)
W3Schools 12600 |
IBM 4500 |
HAVING...
HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.
The syntax for the HAVING function is:
SELECT column,SUM(column) FROM table GROUP BY column HAVING SUM(column) condition value
This "Sales" Table:
Company Amount
W3Schools | 5500 |
IBM | 4500 |
W3Schools | 7100 |
This SQL:
SELECT Company,SUM(Amount) FROM Sales GROUP BY Company HAVING SUM(Amount)>10000
Returns this result
Company SUM(Amount)
W3Schools 12600
The SELECT INTO Statement
The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.
Syntax
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
Make a Backup Copy
The following example makes a backup copy of the "Persons" table:
SELECT * INTO Persons_backup FROM Persons
The IN clause can be used to copy tables into another database:
SELECT Persons.* INTO Persons IN 'Backup.mdb' FROM Persons
If you only want to copy a few fields, you can do so by listing them after the SELECT statement:
SELECT LastName,FirstName INTO Persons_backup FROM Persons
You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:
SELECT LastName,Firstname INTO Persons_backup FROM Persons WHERE City='Sandnes'
Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:
SELECT Employees.Name,Orders.Product INTO Empl_Ord_backup FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID
The SELECT INTO Statement
The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.
Syntax
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
Make a Backup Copy
The following example makes a backup copy of the "Persons" table:
SELECT * INTO Persons_backup FROM Persons
The IN clause can be used to copy tables into another database:
SELECT Persons.* INTO Persons IN 'Backup.mdb' FROM Persons
If you only want to copy a few fields, you can do so by listing them after the SELECT statement:
SELECT LastName,FirstName INTO Persons_backup FROM Persons
You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:
SELECT LastName,Firstname INTO Persons_backup FROM Persons WHERE City='Sandnes'
Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:
SELECT Employees.Name,Orders.Product INTO Empl_Ord_backup FROM Employees INNER JOIN Orders ON Employees.Employee_ID=Orders.Employee_ID
A view is a virtual table based on the result-set of a SELECT statement.
What is a View?
In SQL, a VIEW is a virtual table based on the result-set of a SELECT statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from a single table.
Note: The database design and structure will NOT be affected by the functions, where, or join statements in a view.
Syntax
CREATE VIEW view_name AS SELECT column_name(s) FROM table_name WHERE condition |
Note: The database does not store the view data! The database engine recreates the data, using the view's SELECT statement, every time a user queries a view.
Using Views
A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc., to a view, it allows you to present exactly the data you want to the user.
The sample database Northwind has some views installed by default. The view "Current Product List" lists all active products (products that are not discontinued) from the Products table. The view is created with the following SQL:
CREATE VIEW [Current Product List] AS SELECT ProductID,ProductName FROM Products WHERE Discontinued=No
We can query the view above as follows:
SELECT * FROM [Current Product List]
Another view from the Northwind sample database selects every product in the Products table that has a unit price that is higher than the average unit price:
CREATE VIEW [Products Above Average Price] AS SELECT ProductName,UnitPrice FROM Products WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
We can query the view above as follows:
SELECT * FROM [Products Above Average Price]
Another example view from the Northwind database calculates the total sale for each category in 1997. Note that this view select its data from another view called "Product Sales for 1997":
CREATE VIEW [Category Sales For 1997] AS SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales FROM [Product Sales for 1997] GROUP BY CategoryName
We can query the view above as follows:
SELECT * FROM [Category Sales For 1997]
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
SELECT * FROM [Category Sales For 1997] WHERE CategoryName='Beverages'
Modern SQL Servers are built on RDBMS.
DBMS - Database Management System
A Database Management System (DBMS) is a computer program that can access data in a database. The DBMS program enables you to extract, modify, or store information in a database. Different DBMS programs provide different functions for querying data, reporting data, and modifying data.
RDBMS - Relational Database Management System
A Relational Database Management System (RDBMS) is a Database Management System (DBMS) where the database is organized and accessed according to the relationships between data. RDBMS was invented by IBM in the early 1970's. RDBMS is the basis for SQL, and for all modern database systems like Oracle, SQL Server, IBM DB2, Sybase, MySQL, and Microsoft Access.SQL Quick Reference from W3Schools. Print it, and fold it in your pocket.
SQL Syntax
Statement | Syntax |
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AND / OR | SELECT column_name(s) FROM table_name WHERE | condition AND|OR condition |
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ALTER TABLE (add column) | ALTER TABLE table_name ADD column_name datatype |
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ALTER TABLE (drop column) | ALTER TABLE table_name DROP COLUMN column_name |
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AS (alias for column) | SELECT column_name AS column_alias FROM table_name |
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AS (alias for table) | SELECT column_name FROM table_name AS table_alias |
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BETWEEN | SELECT column_name(s) FROM table_name WHERE | column_name BETWEEN value1 AND value2 |
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CREATE DATABASE | CREATE DATABASE database_name |
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CREATE INDEX | CREATE INDEX index_name ON table_name (column_name) |
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CREATE TABLE | CREATE TABLE table_name ( column_name1 data_type, | column_name2 data_type, ....... ) |
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CREATE UNIQUE INDEX | CREATE UNIQUE INDEX index_name ON table_name | (column_name) |
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CREATE VIEW | CREATE VIEW view_name AS SELECT column_name | (s) FROM table_name WHERE condition |
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DELETE FROM | DELETE FROM table_name (Note: Deletes the entire table!!) or DELETE FROM table_name WHERE condition |
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DROP DATABASE | DROP DATABASE database_name |
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DROP INDEX | DROP INDEX table_name.index_name |
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DROP TABLE | DROP TABLE table_name |
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GROUP BY | SELECT column_name1,SUM(column_name2) FROM | table_name GROUP BY column_name1 |
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HAVING | SELECT column_name1,SUM(column_name2) FROM | table_name GROUP BY column_name1 HAVING SUM | (column_name2) condition value |
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IN | SELECT column_name(s) FROM table_name WHERE | column_name IN (value1,value2,..) |
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INSERT INTO | INSERT INTO table_name VALUES (value1, value2,....) | or INSERT INTO table_name (column_name1, column_name2,...) VALUES (value1, value2,....) |
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LIKE | SELECT column_name(s) FROM table_name WHERE | column_name LIKE pattern |
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ORDER BY | SELECT column_name(s) FROM table_name ORDER | BY column_name [ASC|DESC] |
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SELECT | SELECT column_name(s) FROM table_name |
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SELECT * | SELECT * FROM table_name |
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SELECT DISTINCT | SELECT DISTINCT column_name(s) FROM table_name |
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SELECT INTO (used to create backup copies of tables) SELECT * INTO new_table_name FROM original_table_name or SELECT column_name(s) INTO new_table_name FROM original_table_name
TRUNCATE TABLE (deletes only the data inside the table) TRUNCATE TABLE table_name
UPDATE table_name SET column_name=new_value [,column_name=new_value] WHERE column_name=some_value
WHERE SELECT column_name(s) FROM table_name WHERE condition
SQL Summary
This tutorial has taught you the standard computer language for accessing and manipulating database systems.
You have learned how to execute queries, retrieve data, insert new records, delete records and update records in a database with SQL.
SQL is a standard language that works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, MySQL, Sybase, and other database systems.
Now You Know SQL, What's Next?
The next step is to learn ADO.
ADO is a programming interface to access data in a database from a web site.
ADO uses SQL to query data in a database.
If you want to learn more about ADO, please visit our ADO