What is Database Constraint?

Constraints are declarative rules that the database enforces automatically. NOT NULL prevents missing values. UNIQUE ensures no duplicate values in a column. CHECK validates that values meet a condition (e.g., price > 0). PRIMARY KEY combines NOT NULL and UNIQUE to identify rows. FOREIGN KEY enforces referential integrity between tables. EXCLUDE constraints (PostgreSQL) prevent overlapping ranges. Constraints catch data errors at the database level regardless of which application inserts data, providing a critical safety net that application-level validation alone cannot guarantee.