Primary key, referential integrity, check, and unique constraints

Constraints are rules that limit the values that can be inserted, deleted, or updated in a table.

Primary key constraints

A primary key constraint is a column or combination of columns that has the same properties as a unique constraint. You can use a primary key and foreign key constraints to define relationships between tables.

Referential integrity (or foreign key) constraints

A foreign key constraint (also referred to as a referential constraint or a referential integrity constraint) is a logical rule about values in one or more columns in one or more tables. For example, a set of tables shares information about a corporation's suppliers. Occasionally, a supplier's name changes. You can define a referential constraint stating that the ID of the supplier in a table must match a supplier ID in the supplier information. This constraint prevents insert, update, or delete operations that would otherwise result in missing supplier information.

Check constraints

A (table) check constraint sets restrictions on data added to a specific table.

Unique constraints

A unique constraint (also referred to as a unique key constraint) is a rule that forbids duplicate values in one or more columns within a table. Unique and primary keys are the supported unique constraints.