If k is a foreign key in relation r1, then
? every tuple of r1 has adistinct value for k.
? k cannot have a nullvalue for tuples in r1.
? k is a key for someother relation.
? k is a primary key forr1.

K is a key for some other relation.

Explanation:

  If K is a foreign key in relation R 1, then K is a key for some other relation as foreign key is the key which provide index relation table. It basically declared index of one table are related to another and place the constraint. And in the relation operator that comprises its value from all the rows in the table and it is used to prevent from destroy between the tables.

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