SQL for QA and Testing Engineer

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What is SQL ?
SQL stands for Structured Query Language.It is a language that provides an interface to relational database systems. The pronunciation of SQL is "ess cue ell," and not "sequel" .SQL was developed by IBM in the 1970s for use in System R, and is a de facto standard, as well as an ISO and ANSI standard.
 SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures.
The development of SQL is governed by standards. A major revision to the SQL standard was completed in 1992, called SQL2. SQL3 support object extensions and are (partially?) implemented in Oracle8 and 9i.

What are the differences between DDL, DML and DCL commands?
DDL - Stands for Data Definition Language: statements used to define the structure of database or schema. examples

§  CREATE - use to create Table in the database
§  ALTER - use to alters the Table structure of the database
§  DROP - use to delete tables from the database.
§  TRUNCATE - use to remove all records from a table, including all spaces allocated for the records 
§  COMMENT - use to add comments to the data dictionary
§  RENAME - use to rename an object
DML - Stands for Data Manipulation Language: statements used for retrieving and updating data within schema objects.  examples:
§  SELECT - use to  retrieve data from the a database
§  INSERT - use to insert data into a table
§  UPDATE - use to  updates existing data within a table
§  DELETE - use to deletes all records from a table, the space for the records remain

DCL - Stands for Data Control Language. 
 examples:
§  GRANT - it gives user's access privileges to database
§  REVOKE - use to withdraw access privileges given with the GRANT command
TCL - It stands for Transaction Control: statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.
examples
§  COMMIT - Use to save work done
§  SAVEPOINT - use to identify a point in a transaction to which you can later roll back
§  ROLLBACK - use to restore database to original since the last COMMIT
§  SET TRANSACTION - use to change transaction options like isolation level and what rollback segment to use
Note:-DML are not auto-commit. i.e. you can roll-back the operations, but DDL are auto-commit


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