🔒 SQL Constraints

Constraints enforce rules at the table/column level — stopping invalid, duplicate, or missing data before it even hits your database.

✅ Common Constraint Types:

• PRIMARY KEY — Unique identifier for each row
• UNIQUE — Ensures all values in a column are different
• NOT NULL — Prevents NULL (empty) values
• CHECK — Limits values based on a condition
• FOREIGN KEY — Connects records between tables

📖 Example: Creating a Table with Constraints