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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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