What does a referential integrity constraint with a foreign key enforce?

What does a referential integrity constraint with a foreign key enforce?

• A. In each row, the foreign key value either matches a primary key value in the referred relation or is null
• B. The foreign key values must be unique
• C. Every table must have a primary key
• D. The foreign key must be indexed

Answer: (A)

Referential integrity with a foreign key ensures that a value in the child table points to a valid row in the parent table. In practice, every foreign key value must either match an existing primary key value in the referenced table or be NULL if the column allows it. This keeps relationships consistent across tables and prevents references to non-existent rows, such as an order referencing a non-existent customer. It also means foreign keys do not have to be unique—multiple child rows can reference the same parent. While performance can benefit from indexing a foreign key, enforcing the constraint itself does not require an index, and it does not by itself mandate that every table has a primary key.

Referential integrity with a foreign key ensures that a value in the child table points to a valid row in the parent table. In practice, every foreign key value must either match an existing primary key value in the referenced table or be NULL if the column allows it. This keeps relationships consistent across tables and prevents references to non-existent rows, such as an order referencing a non-existent customer. It also means foreign keys do not have to be unique—multiple child rows can reference the same parent. While performance can benefit from indexing a foreign key, enforcing the constraint itself does not require an index, and it does not by itself mandate that every table has a primary key.