Database Languages

The five categories

Every command in MySQL falls into one of five categories, grouped by what kind of thing the command actually does.

DDL — Data Definition Language

Commands related to defining the structure of a table, not the data inside it.

CREATE TABLE, DROP, ALTER

DML — Data Manipulation Language

Commands used for manipulating the actual data inside a table.

INSERT, UPDATE, DELETE

DCL — Data Control Language

Commands for controlling access, who's allowed to do what.

GRANT, REVOKE

TCL — Transaction Control Language

Commands for managing transactions, groups of operations meant to succeed or fail as one unit.

• A transaction is a complete unit of work in a database, potentially several statements bundled together as a single logical operation.
• A transaction has exactly two outcomes: successful, every operation in it went through, or never initiated, as if none of it happened. There's no partial state in between, a transaction can't be "60% done", it's either all the way through or rolled back entirely, atomicity, not a gradual progress bar.
• COMMIT makes the transaction's changes permanent.
• ROLLBACK undoes the transaction's changes, reverting back to before it started.

DQL — Data Query Language

Commands for reading data back out.

SELECT

SELECT gets grouped separately as its own category, DQL, even though older classifications sometimes lump it under DML alongside INSERT/UPDATE/DELETE. It doesn't change data the way those three do, it only reads it, which is the whole reason it earns a category of its own.