Q.What is a SQL ? 

      SQL is not a database management system but it is a language and a tool for communicating with the DBMS. SQL stands for Structured Query Language which organizes, manages and retrieves data from the database.It is a non procedural language. Today it is the most standard computer database language.

Q. What is DDL, DML, DCL and TCL command ?

      DDL stands for Data Definition Language.It helps in defining the structure of
database objects. Basically it creates or removes or changes the database objects.
Examples: ALTER,CREATE, DROP,TRUNCATE.

       DML stands for Data Manipulation Language. It helps in manipulating or managing the data in the database. Basically it helps in retrieving,storing modifying and deleting data in the database.
Examples: SELECT, INSERT, UPDATE, DELETE.

       DCL stands for Data Control Language. It helps in controlling access to data stored in database thus securing it.

Examples: GRANT, REVOKE.

       TCL stands for Transaction Control Language. As the name suggests, it helps in managing different transactions occurring in database.

Examples: COMMIT, ROLLBACK .