SQL commands are grouped by what they act on — the structure, the data, permissions, or transactions. A classic first interview question.

Group	Stands for	Commands	Acts on
DDL	Data Definition Language	`CREATE`, `ALTER`, `DROP`, `TRUNCATE`, `RENAME`	Schema objects (tables, indexes, views)
DML	Data Manipulation Language	`INSERT`, `UPDATE`, `DELETE`, `MERGE`	Rows inside tables
DQL	Data Query Language	`SELECT`	Reading rows (often counted as part of DML)
DCL	Data Control Language	`GRANT`, `REVOKE`	Permissions of users and roles
TCL	Transaction Control Language	`COMMIT`, `ROLLBACK`, `SAVEPOINT`, `BEGIN`	Grouping DML into atomic units

One of each

SQL

-- DDL: define structure
CREATE TABLE notes (id INTEGER PRIMARY KEY, body TEXT NOT NULL);

-- TCL + DML: change data inside a transaction
BEGIN;
INSERT INTO notes (body) VALUES ('learn SQL'), ('practice joins');
UPDATE notes SET body = 'practice window functions' WHERE id = 2;
COMMIT;

-- DQL: read it back
SELECT id, body FROM notes ORDER BY id;

Output

id | body
---+--------------------------
1  | learn SQL
2  | practice window functions

PostgreSQL / MySQL / SQL Server

-- DCL: permissions (server databases only; SQLite has no users)
GRANT SELECT, INSERT ON notes TO analyst;
REVOKE INSERT ON notes FROM analyst;

Why the grouping matters

Is TRUNCATE DDL or DML?

Is SELECT part of DML?