Constraint

The relational model defines many constraints in order to control data integrity, redundancy, and validity. Here are some examples of checking for data:

• Redundancy: Duplicate tuples are not allowed in the relation.
• Validity: Check constraints and domain constraints are used to validate the data input, for example, the date of birth should be a date that occurred in the past.
• Integrity: The relations within a single database are linked to each other. An action on a relation such as updating or deleting a tuple might leave the other relations in an invalid state.

We could classify the constraints in a relational database roughly into two categories:

• Inherited constraints from the relational model: Domain integrity, entity integrity, and referential integrity constraints.
• Semantic constraint, business rules, and application-specific constraints: These constraints cannot be expressed explicitly by the relational model. However, with the introduction of procedural SQL languages such as PL/pgSQL for PostgreSQL, relational databases can also be used to model these constraints.