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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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This is another example of a nested query where we have two SQL SELECT statements, one nested inside the other. This one is a little different from the simpler example in Listing 4-5. The WHERE condition in the inner query refers to part of the row being considered in the outer query, that is, e.MemberID = m.MemberID. I find the easiest way to interpret Listing 4-10 is with reference to Figure 4-4. The inner query is looking for a row in the Entry table with the same value for MemberID as the row under consideration in the Member table. If such a row or several such rows EXIST, then we are in business.
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For those of you who are thinking that this seems like a complicated way to get a simple result, you are right (partly). The query in Listing 4-10 retrieves the same members as an inner join (on MemberID) between Member and Entry. However, what if we want those members who have NOT entered a tournament This requires only a tiny change to our relational calculus and SQL expressions. Instead of looking for members where a matching row in Entry exists, we now want those where a matching row does NOT exist. Adding the word NOT to Listing 4-10 gives us what we require, as shown in Listing 4-11.
Listing 4-11. SQL Statement to Retrieve Members Who Have NOT Entered a Tournament SELECT m.Lastname, m.FirstName FROM Member m WHERE NOT EXISTS (SELECT * FROM Entry e WHERE e.MemberID = m.MemberID)
The NOT EXISTS construction will look through every row in the Entry table checking whether there is a row matching the current row in the Member table. The names of the member will be retrieved only if no matching row is found. Now we have enough ammunition to tackle the query about members who have not entered an Open tournament. As always, there are several ways we can express the SQL query, and one way appears in Listing 4-12. It is almost the same as Listing 4-11, but instead of looking for members with no matching records in the Entry table, we look for members who do not have a matching record in the bottom section of the table shown in Figure 4-2. That set of rows results from joining Entry and Tournament and selecting just the rows for Open tournaments.
Listing 4-12. SQL Statement to Retrieve Members Who Have NOT Entered an Open Tournament SELECT m.Lastname, m.FirstName FROM Member m WHERE NOT EXISTS (SELECT * FROM Entry e INNER JOIN Tournament t ON e.TourID = t.TourID WHERE e.MemberID = m.MemberID and t.TourType = 'Open')
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