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13.8 Relvar SPJ′ from the section “Redundancy Free Normal Form” was subject to what might be called a
“symmetric” JD─viz., the JD {{SNO,PNO},{PNO,JNO},{JNO,SNO}}─and yet displayed some asymmetry also,
in that just one of the three components of that JD corresponded to a key. Intuitively, you might expect the other
two components to correspond to keys as well. Show this isn't necessarily so.
13.9 Design a database for the following. The entities to be represented are soccer match fixtures for a certain
team. For matches that have already been played, we wish to record “goals for” and “goals against”; however,
these two properties clearly make no sense for matches that have yet to be played. What normal forms are your
relvars in?
13.10 Let relvar SCP have attributes SNO, PNO, and CITY and predicate Supplier SNO and part PNO are both
located in city CITY . Can SCP be derived from our usual S, P, and SP relvars? What normal form is it in? Can you
think of any conventional wisdom this example might fly in the face of?
13.11 Define DK/NF. Give an example of a relvar in 6NF that's not in DK/NF.
13.12 What's the difference between SKNF and overstrong PJ/NF? In fact, is there a difference?
13.13 Give definitions, as precise as you can make them, of the relational operators restriction and union.
13.14 In the body of the chapter, I showed informally how reducing a relvar to 6NF projections corresponded to
reducing a conjunctive predicate to simple predicates. Could there be such a thing as a disjunctive predicate? How
might a relvar correspond to such a predicate? What would be involved in reducing such a predicate to simple
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