What are SQL constraints?

SQL constraints are rules applied to columns in a database table to limit the type of data that can be stored. They help enforce data integrity and accuracy. Constraints can be defined when the table is created using CREATE TABLE, or added later using ALTER TABLE.

What is the difference between PRIMARY KEY and UNIQUE constraints?

A PRIMARY KEY constraint uniquely identifies each row and does not allow NULL values. A UNIQUE constraint also enforces uniqueness but allows one NULL value. A table can have only one primary key, but multiple unique constraints.

What is a FOREIGN KEY constraint in SQL?

A FOREIGN KEY constraint ensures referential integrity by requiring that the value in a column matches a value in the primary key column of another (referenced) table. It prevents orphaned records and maintains valid relationships between tables.

When should I use a CHECK constraint?

Use a CHECK constraint when you want to ensure that all values in a column satisfy a specific condition, such as a price being greater than zero or an age being between 18 and 120. It runs a validation expression for every INSERT and UPDATE.

SQL CONSTRAINT

SQL > Constraint

You can place constraints to limit the type of data that can go into a table. Such constraints can be specified when the table when the table is first created via the CREATE TABLE statement, or after the table is already created via the ALTER TABLE statement.

SQL constraints are rules that enforce data integrity by restricting the type and format of data stored in a table column. They can be defined at table creation or added later via ALTER TABLE.

Common types of constraints include the following:

NOT NULL Constraint: Ensures that a column cannot have NULL value.

DEFAULT Constraint: Provides a default value for a column when none is specified.

UNIQUE Constraint: Ensures that all values in a column are different.

CHECK Constraint: Makes sure that all values in a column satisfy certain criteria.

Primary Key Constraint: Used to uniquely identify a row in the table.

Foreign Key Constraint: Used to ensure referential integrity of the data.

Each constraint is discussed in the following sections.

Frequently Asked Questions

What are SQL constraints?: SQL constraints are rules applied to table columns that limit the type or range of data that can be stored. They ensure data accuracy and integrity. Common constraints include NOT NULL, DEFAULT, UNIQUE, CHECK, PRIMARY KEY, and FOREIGN KEY. They can be added when a table is created or via ALTER TABLE afterward.
What is the difference between PRIMARY KEY and UNIQUE constraints?: A PRIMARY KEY uniquely identifies each row and does not permit NULL values — a table can have only one primary key. A UNIQUE constraint also enforces uniqueness but allows one NULL value, and a table can have multiple UNIQUE constraints on different columns.
What is a FOREIGN KEY constraint in SQL?: A FOREIGN KEY constraint maintains referential integrity between two tables by requiring that a column's value matches a value in the referenced table's primary key. This prevents orphaned records and ensures related data remains consistent.
When should I use a CHECK constraint?: Use a CHECK constraint to validate that column values meet specific criteria — for example, ensuring a salary column is greater than zero, or a status column only contains allowed values. CHECK constraints run on every INSERT and UPDATE operation.

Next: SQL NOT NULL

This page was last updated on March 19, 2026.

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SQL CONSTRAINT

Frequently Asked Questions

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