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Database Design and Relational Theory by C.J. Date

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CHAPTER 13

13.1 See Figure 13-1.

13.2 See the body of the chapter.

13.3 Irreducibility of keys and FDs, and the relevance of FD irreducibility to 2NF, are all discussed in Chapter 4; FD irreducibility is discussed further in Chapter 5. Irreducible covers are discussed in Chapter 6. Irreducible JDs are discussed in Chapter 11. Irreducible (i.e., 6NF) relvars and the associated notion of “irreducible facts” are discussed in the body of the present chapter (i.e., Chapter 13).

13.4 See the body of the chapter.

13.5 The main point that occurs to me is that it might be nice to have some kind of “master” relvar whose primary purpose is just to record the part numbers for all parts currently represented in the database. If we call that relvar P, there’ll be EQDs between that relvar P and the projection on {PNO} of each of the relvars PN, PL, PW, and PC (instead of EQDs between, arbitrarily, the projection of PN on {PNO} and the projection on {PNO} of PL, PW, and PC; indeed, one nice thing about having the master relvar is precisely that it avoids that slight arbitrariness).

Moreover, suppose every part always has a known name and weight but doesn’t necessarily have a known color or city. Then we can combine relvars P, PN, and PW, making that combination—which I’ll still call P—the master relvar, and replace the EQDs previously required by foreign key constraints from PL and PC to that master relvar. (A part with no known color will be represented in P but not PL; likewise, a part with no known ...

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