Introduction to SQL

SQL (Structured Query Language) is how you talk to a relational database — asking it to store, find, change, or remove data. Every SQL statement falls into one of four families, which is exactly how this section is organized, lesson by lesson.

What SQL Actually Does

SQL is a declarative language: you describe the result you want, not the steps to get it. The database engine decides how to actually fetch it.

SELECT name FROM Employee WHERE department = 'IT';

You state the "what" (employees in IT), not the "how" (loop, compare, collect) — the engine works that part out.
Every statement ends in a semicolon.

Common Mistakes

Forgetting the semicolon in multi-statement scripts.
Assuming SQL runs top-to-bottom like a normal program — it describes a result, it doesn’t execute as a sequence of steps the way a script does.

The Four SQL Families

Every SQL command belongs to one of four families, grouped by what they affect:

Family	Full name	Affects	Example commands
DDL	Data Definition Language	The structure — tables, columns	CREATE, ALTER, DROP, TRUNCATE
DML	Data Manipulation Language	The data inside that structure	SELECT, INSERT, UPDATE, DELETE
DCL	Data Control Language	Who can touch it	GRANT, REVOKE
TCL	Transaction Control Language	When changes stick	COMMIT, ROLLBACK, SAVEPOINT

Diagram showing the four SQL command families — DDL, DML, DCL, and TCL — each with its full name and example commands

The Mistake Worth Remembering Most

Mixing up DELETE (DML, removes rows) with DROP (DDL, removes the whole table) — one of the most common beginner mistakes in SQL, and one of the most costly to get wrong.

This section walks through each family in its own lesson, plus the querying, functions, and advanced techniques built on top of them. Next up: databases and tables — the structure everything else in SQL operates on.