atabase constraints must follow relvar constraints
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The text below is from "Introduction to Database Systems, J. Date". I can't the bolded part. why since Relvar constraints must obviously be checked immediately, must database constraints be checked immediately too? Why database constraints must follow relvar constraints, please? The previous edition of this book stated that relvar constraints were checked immediately but database constraints were checked at end-of-transaction (a position that many writers concur with, though they usually use different terminology). But The Principle of Interchangeability (of base and derived relvars--see Chapter 9) implies that the very same real-world constraint might be a relvar constraint with one design for the database and a database constraint with another! Since relvar constraints must obviously be checked immediately, it follows that database constraints must be checked immediately too. .