Discussion
Among the following, which is foreign key constraint ?
Answer: C
The foreign key constraint is referential integrity.
Referential integrity requires that a foreign key value in one relation either matches an existing primary key value in the relation it refers to, or is null. It stops a row from referring to a record that does not exist.
Domain integrity restricts attribute values to their allowed domain. Entity integrity requires primary key values to be unique and not null. Key integrity concerns uniqueness of keys within a relation. None of these is the constraint a foreign key enforces.
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