What is a Constraint in a database?

A constraint in a database is a rule applied to data in a table to ensure its integrity and correctness. Constraints help prevent the entry of incorrect or conflicting data.

Main types of constraints:

• PRIMARY KEY — a unique identifier for a row, cannot be NULL.
• FOREIGN KEY — ensures referential integrity between tables.
• UNIQUE — guarantees the uniqueness of values in a column.
• NOT NULL — prohibits NULL values in a column.
• CHECK — sets a condition that values must satisfy.

For example, a NOT NULL constraint on the email field guarantees that each customer will have an email specified.