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A condition that data held in a relational database must satisfy in order to be valid. Constraints are enforced by the database management system, and any attempted changes to data that would violate a constraint are rejected. Types of constraint include: primary keys; referential integrity; forbidding null values in a field; specifying that a field's values must be different in every row of a table; and arbitrary logical expressions, constructed from the values of one or more fields in the same row, that must evaluate to true.

Subjects: Computing.

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