Integrity Constraints

An integrity constraint allows us to place constraints on data being entered into/updated in a relation.

They guard against accidental damage to the database, by ensuring that authorized changes to the database do not result in a loss of data consistency.

Some common integrity constraints:

• NOT NULL: the value for the attribute must not be null

• UNIQUE: the value of the attribute must be unique. The specification UNIQUE($A_1, A_2, ..., A_n$) indicates that ($A_1, A_2, ..., A_n$) form the candidate key
• PRIMARY KEY

• CHECK (P): P is a predicate; used to check if a condition is satisfied

• Referential Integrity: this is needed when using foreign keys. It ensures that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation