Discussion
Which of the following is true about "referential integrity" ?
Answer: A
Referential integrity ensures that foreign keys are valid and correspond to primary keys.
It requires every non-null foreign key value in one table to match an existing primary key value in the table it refers to, so there are no orphan records, such as an order for a customer who does not exist.
Preventing nulls is the NOT NULL constraint, preventing duplicates is the job of primary key or UNIQUE constraints, and referential integrity is a central part of the relational model.
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