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DB Tables which are in 3NF or 4NF but not in DKNF

Posted on 2009-04-08
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Last Modified: 2013-12-25
Are there examples of Relational tables which are in 3NF or 4NF but not in Domain Key Normal Form?
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Question by:dtivmk
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dportas earned 500 total points
ID: 24102146
Yes. DKNF is unlike other normal forms because it is concerned with constraints rather than purely FDs, MVDs and JDs. A relation schema is in DKNF if all its constraints are equivalent to its key and domain constraints only. According to Fagin (who invented DKNF) a DKNF schema is automatically in 5NF but a 5NF schema is not necessarily in DKNF.

A trivial example of a relation in 5NF but not in DKNF is:
R{A,B} KEY {A,B}
with an inclusion dependency between A and B. In other words the constraint is:
project{A}( R ) = project{B}( R )

This constraint isn't implied by the key {A,B} therefore R is not in DKNF.
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by:dportas
ID: 24102257
I thought of an even neater example:

P {A}
constraint: SUM(A)< 10

ie. The sum total of A must be less than 10
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