martes, 13 de enero de 2009

Diferencia Entre DDL, DML and DCL commands?

1)Query-select

2)DDL - Data Definition Language: statements used to define the database structure or schema. Some examples:
* CREATE - to create objects in the database
* ALTER - alters the structure of the database
* DROP - delete objects from the database
* TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
* COMMENT - add comments to the data dictionary
* RENAME - rename an object

3)DML - Data Manipulation Language: statements used for managing data within schema objects. Some examples:
* SELECT - retrieve data from the a database
* INSERT - insert data into a table
* UPDATE - updates existing data within a table
* DELETE - deletes all records from a table, the space for the records remain
* MERGE - UPSERT operation (insert or update)
* CALL - call a PL/SQL or Java subprogram
* EXPLAIN PLAN - explain access path to data
* LOCK TABLE - control concurrency

4)DCL - Data Control Language. Some examples:

* GRANT - gives user's access privileges to database
* REVOKE - withdraw access privileges given with the GRANT command

5)TCL - Transaction Control: statements used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.
* COMMIT - save work done
* SAVEPOINT - identify a point in a transaction to which you can later roll back
* ROLLBACK - restore database to original since the last COMMIT
* SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use

6)System Control Statements
These statements change the properties of the Oracle database instance. The only system control statement is ALTER SYSTEM. It lets users change settings, such as the minimum number of shared servers, kill a session, and perform other tasks.

7)Embedded SQL StatementsThese statements used in a procedural language program, such as those used with the Oracle precompilers. Examples include OPEN, CLOSE, FETCH, and EXECUTE.