DDL & DML: The Two Halves of SQL

What is DDL?

DDL stands for Data Definition Language. These commands define or change the structure of a database - tables, columns, and their types - without touching the actual data inside.

Example - creating a table:

Adding a column to an existing table:

What is DML?

DML stands for Data Manipulation Language. These commands work with the actual data inside the tables - inserting, updating, deleting, or retrieving rows.

Example - inserting a row:

Updating a row:

Deleting a row:

Retrieving data:

Key Difference

The distinction that clarified this for me: DDL changes the container, DML changes what's inside the container.

• Running CREATE TABLE builds an empty structure - no data exists yet. Only after that does INSERT (a DML command) actually put rows into it.
• Most DDL commands are harder to undo. DROP TABLE deletes the table and everything in it, structure included - there's no "undo" in most database systems once it's committed.
• DML changes like DELETE or UPDATE are often reversible if wrapped in a transaction, but DDL is generally treated as a bigger, more permanent action.

What I Learned

Understanding that DDL commands run first, and rarely change once a table is in use, helped me realize why planning a table's structure carefully upfront (column types, primary keys) matters - going back to change it later with ALTER is possible, but riskier once real data and other queries already depend on that structure.