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Key Constraints in Relational Database Model

Key constraint is implied by the existence of candidate keys. The intension includes a specification of the attribute(s) consisting the primary key and specification of the attribute(s) consisting alternate keys, if any. Each of these specifications implies a uniqueness constraint(by definition of candidate key);in addition primary key specification implies a no-nulls constraint( by integrity rule 1).

Referential constraints

Referential constraints are constraints implied by the existence of foreign keys. The intension includes a specification of all foreign keys in the relation. Each of these specifications implies a referential constraint ( by integrity rule 2).

Other constraints

Many other constraints are possible in theory.
Examples
salary >= 10000

See Also


  

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