Backend Development › Relational Databases & SQL

DDL, DML, DCL and DQL

SQL's categories: defining structure, changing data, granting access, querying.

Also known as: DDL, DML, DCL, DQL

SQL statements are often grouped into four categories, each for a different job:

• DDL (Data Definition Language): changes structure. CREATE TABLE, ALTER TABLE, DROP TABLE.
• DML (Data Manipulation Language): changes data. INSERT, UPDATE, DELETE.
• DCL (Data Control Language): controls access. GRANT and REVOKE.
• DQL (Data Query Language): reads data. SELECT.

CREATE TABLE books (id INTEGER PRIMARY KEY, title TEXT NOT NULL);   -- DDL
INSERT INTO books (id, title) VALUES (1, 'Dune');                   -- DML
GRANT SELECT ON books TO reporting_user;                            -- DCL
SELECT title FROM books;                                            -- DQL

Some sources group SELECT under DML, so don’t be surprised by either grouping. The categories are a way to talk about what a statement does.

The classic mistake is running a DDL statement casually. Changing a table’s structure can affect every query and application that uses it. Whether a DDL statement can be rolled back depends on the database, so check before running one, and prefer a migration you can review and reproduce. The CRUD operations are the everyday part of this work.