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Let s recall some relational calculus from the previous chapter. Our questions about who did or did not enter Open tournaments can be conveniently represented with calculus expressions. This approach allows us to answer the questions involving negatives that we had trouble with in the previous section. Let s start with a simple question. For example: What are the names of all members who have ever entered any tournament We can start by thinking in terms of which rows of the Member table would satisfy our question. Consider the following sentence and Figure 4-4 together: I ll write out the names from row m, where m comes from the Member table, if there exists a row e in the Entry table where m.MemberID = e.MemberID. Listing 4-9 shows the equivalent calculus expression.
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Listing 4-9. Relational Calculus Expression to Retrieve Members Who Have Entered a Tournament {m.LastName, m.FirstName | Member(m) and (e) Entry(e) and m.MemberID = e.MemberID}
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CHAPTER 4 NES TED QUER IES
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a) Member (some columns)
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b) Entry
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Figure 4-4. William Cooper has entered a tournament because a matching row exists in the
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Entry table.
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We can translate this almost directly into SQL with the use of the keyword EXISTS. Have a look at the SQL statement in Listing 4-10.
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Listing 4-10. SQL Statement to Retrieve Members Who Have Entered a Tournament SELECT m.LastName, m.FirstName FROM Member m WHERE EXISTS (SELECT * FROM Entry e WHERE e.MemberID = m.MemberID)
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