SLIDE Structured Query Language (50 TRANG) SQL commands can be classified in to three types: Data Definition Language commands (DDL) Data Manipulation Language commands (DML) Data Control Language commands (DCL) DDL commands are used to define a database, including creating, altering, and dropping tables and establishing constraints. DML commands are used to maintain and query a database, including updating, inserting, modifying, and querying data. DCL commands are used to control a database including administering privileges and saving of data.