NWPGCL — Assistant Engineer (CSE) Written Test: 2018প্রশ্ন ও সমাধান

DDL, DML, and DCL are the three broad categories of SQL commands, split by what they operate on:

Category	Full form	Purpose	Common commands
DDL	Data Definition Language	Defines/alters the database schema/structure (tables, indexes, views)	CREATE, ALTER, DROP, TRUNCATE, RENAME
DML	Data Manipulation Language	Manipulates the data stored inside existing structures	SELECT, INSERT, UPDATE, DELETE, MERGE
DCL	Data Control Language	Controls access rights/permissions on the database	GRANT, REVOKE
(often added) TCL	Transaction Control Language	Manages transactions (grouping DML statements)	COMMIT, ROLLBACK, SAVEPOINT

• DDL changes are usually auto-committed (cannot be rolled back in most RDBMSs) — e.g., CREATE TABLE Student(...), ALTER TABLE Student ADD Email VARCHAR(50), DROP TABLE Student.
• DML changes affect rows and, importantly, can be rolled back if wrapped in a transaction — e.g., INSERT INTO Student VALUES (1,'Rahim'), UPDATE Student SET Email='x@y.com' WHERE ID=1, DELETE FROM Student WHERE ID=1.
• DCL manages who can do what — e.g., GRANT SELECT, INSERT ON Student TO 'app_user', REVOKE DELETE ON Student FROM 'app_user'.

graph LR
    SQL[SQL Commands] --> DDL["DDL: schema - CREATE/ALTER/DROP"]
    SQL --> DML["DML: data - SELECT/INSERT/UPDATE/DELETE"]
    SQL --> DCL["DCL: permissions - GRANT/REVOKE"]
    SQL --> TCL["TCL: transactions - COMMIT/ROLLBACK"]

Exam takeaway: DDL = structure, DML = data, DCL = permissions, TCL = transaction control — remember by what each command changes: the schema, the rows, or who's allowed to touch them.