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CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
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Example 5: Now I d like to extend Example 4 slightly in order to make an additional point. Suppose relvar EMP does include at least one additional attribute after all, JOB, such that a given employee is a programmer, and is represented in relvar PGMR, if and only if the JOB value in that employee s tuple in EMP has the value Pgmr (maybe other values of JOB Janitor , for example correspond to other relvars). This kind of situation is not at all uncommon in practice, by the way. Once again, then, the design exhibits some redundancy, because it satisfies the following constraint (as well as many similar ones): CONSTRAINT PGMR_CONSTRAINT ( EMP WHERE JOB = 'Pgmr' ) { E# } = PGMR { E# } ; Example 6: In his RM/T paper ( Extending the Database Relational Model to Capture More Meaning, ACM TODS 4, No. 4, December 1979), Codd proposed a certain design discipline, a simplified form of which can be described thus:18 Let E be an entity type, and let ID be a data type such that every entity of type E has exactly one primary identifier (my term, not Codd s), of type ID. For example, E and ID might be the entity type suppliers and the data type supplier numbers, respectively. Let P1, P2, ..., Pn be a set of property types such that every entity of type E has at most one property of each of the types P1, P2, ..., Pn. In the case of suppliers, for example, P1, P2, ..., Pn might be the types name, status, and city (so n = 3 in this example). Note: I m assuming here that a given supplier can have any subset of the three properties, including the empty set in particular (of course!). Then the database should include:
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Exactly one E-relvar, containing ID values for those entities of type E extant at any given time Exactly one P-relvar for each Pi (i = 1, 2, ..., n), containing ID-value:Pi-value pairs for each entity of type E that is extant at any given time and has a property of type Pi at that time
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Applying this RM/T discipline (as I ll call it) to the case of suppliers specifically, we obtain a design that looks like this: VAR S BASE RELATION { S# S# } KEY { S# } ; VAR SN BASE RELATION { S# S#, SNAME NAME } KEY { S# } FOREIGN KEY { S# } REFERENCES S ; VAR ST BASE RELATION { S# S#, STATUS INTEGER } KEY { S# } FOREIGN KEY { S# } REFERENCES S ;
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18. Details of the simplifications aren t important for present purposes. Once again, however, I feel obliged to apologize, slightly, for my use of the vague terms entity and property but here I m simply following the usage in Codd s paper.
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CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
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VAR SC BASE RELATION { S# S#, CITY CHAR } KEY { S# } FOREIGN KEY { S# } REFERENCES S ; Each of these relvars is irreducible, or equivalently in sixth normal form, 6NF (see Example 11 later in this section for further discussion of this concept). Figure 12-12 shows a set of sample values. Note: Those values aren t meant to be the same as our usual sample values, though they re close. Observe in particular that (a) supplier S3 has no status, (b) supplier S4 has no status and no city, and (c) supplier S5 has no name, no status, and no city.
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19. I have in mind here, primarily, a system implemented using the facilities of The TransRelationalTM Model. You can find a preliminary description of this model in Appendix A of my book An Introduction to Database Systems, 8th edition (Addison-Wesley, 2004).
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