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It defines that the tuple in one relation that refers to another relation must refer to an existing tuple in that relation. This constraint is states on two relations (not essentially distinct). It uses a concept of foreign key and has been dealt with in more factors in the next unit.
Example
In the example given above, the value of C^ in every R tuple is matching with the value of C# in some S tuple. If a tuple has values (A6, B2, and C4) is added then it is invalid as referenced relation S doesn't contain C4. Therefore, it will be a violation of referential integrity constraint.
each publisher has uniqe name a mial addrs are also kept on each pubisher.a publisher publish one book.a book publish y one publisher.book identified by isbn.each book writn by one
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I need to Map an E-R diagram to a relational schema
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Benefits of using heuristics
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