What do you mean by DDL, DML, DCL and TCL? Explain.

DDL, DML, DCL, and TCL are types of SQL (Structured Query Language) commands. SQL is a standard language for managing data held in a relational database management system.

1. DDL (Data Definition Language): DDL is used to define the structure that holds the data. For example, to create, alter, truncate, and drop database and tables. DDL is not used to manipulate data.

   • CREATE: To create databases and tables.
   • ALTER: For altering existing database structures.
   • DROP: To drop databases and tables.
   • TRUNCATE: To remove all records from a table, including all spaces allocated for the records are removed.

2. DML (Data Manipulation Language): DML is used for manipulation of the data itself. Typical examples of DML include INSERT, UPDATE, and DELETE statements.

   • INSERT: To insert data into tables.
   • UPDATE: To update existing data within a table.
   • DELETE: To delete records from a database table.

3. DCL (Data Control Language): DCL is used to control the visibility of data like granting database access and set privileges to create tables, etc. It includes commands like GRANT and REVOKE.

   • GRANT: To allow users access privileges to the database.
   • REVOKE: To take back permissions from the user.

4. TCL (Transaction Control Language): TCL commands deal with the transaction operations of the database. These are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.

   • COMMIT: To write and store the changes to the database.
   • ROLLBACK: To restore the database to the original since the last COMMIT.
   • SAVEPOINT: Creates points within groups of transactions in which to ROLLBACK.
   • SET TRANSACTION: Places a name on a transaction.

Each of these command types plays a crucial role in communicating and managing databases.