MySQL Constraints Explained: PRIMARY KEY, UNIQUE, CHECK & More

Constraints are rules enforced at the database level so invalid data can never sneak in. For analysts, constraints are the reason you can trust the numbers in your reports — the database guarantees integrity before analysis even begins.

Key Points

• PRIMARY KEY: uniquely identifies every row; no NULLs, no duplicates
• FOREIGN KEY: links a row to a parent table, preserving referential integrity
• UNIQUE: no duplicate values allowed (e.g., email addresses)
• NOT NULL: the column must always have a value
• DEFAULT: fills in a value automatically when none is provided
• CHECK (MySQL 8.0+): validates conditions like salary > 0