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Listing 4-24. Deleting Entries from Tournament 25, 2007, for Low Handicap Members DELETE FROM Entry WHERE TourID = 25 AND Year = 2007 AND MemberID IN (SELECT MemberID FROM Member WHERE Handicap < 20)
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We can use nested queries along with the keywords IN and EXISTS in many situations. Many nested queries can be written in alternative ways. In 9, we will look at performance issues relating to different ways of expressing queries, but in general you should use the way that feels most natural to you when designing a query. The following are the main types of nested queries:
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Nested Query
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Replacing a single value with a subquery that returns a single value
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SELECT * FROM Entry e WHERE e.MemberID = 235 The previous can be replaced with this: SELECT * FROM Entry e WHERE e.MemberID = (SELECT m.MemberID FROM Member m WHERE m.LastName = 'Cooper') SELECT * FROM Entry e WHERE e.TourID IN (36,38,40) The previous can be replaced with this: SELECT * FROM Entry e WHERE e.TourID in (SELECT t.TourID FROM Tournament t WHERE t.TourType = 'Open') SELECT * FROM Member m WHERE EXISTS (SELECT * FROM Entry e WHERE e.MemberID = m.MemberID) Show us members who have a corresponding row in the Entry table.
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Replacing a set of values with a subquery returning a set of single values
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Checking for the existence of rows
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Nested queries can be used in many situations, including the following:
Task
Comparing values with the results of aggregates Constructing queries with negatives Making changes to table data
Example
SELECT * FROM Member m WHERE m.Handicap < (SELECT AVG(Handicap) FROM Member) Find members with handicaps less than the average. SELECT * FROM Member m WHERE NOT EXISTS (SELECT * FROM Entry e WHERE e.MemberID = m.MemberID) Show us members who have NOT entered a tournament. INSERT INTO Entry (MemberID, TourID, Year) SELECT MemberID, 25, 2007 FROM Member WHERE MemberType = 'Junior' Add a row in the Entry table for every junior for tournament 25 in 2007.
Self Joins
n 2, we looked at simple queries on a single table. In this chapter, we ll explore more complex queries on a single table in particular, those requiring us to look at more than one row of the table at a time. The two types of queries we will look at are those where the table is involved in a self relationship and queries to answer questions such as Which members entered both these tournaments
Self Relationships
Let s add some more information to our Member table. Suppose some members have coaches assigned to them. How do we represent that in the class diagrams we talked about in 1 We could take the approach shown in Figure 5-1 with two classes: Member and Coach. Recall what the lines and numbers mean. From left to right, a coach might have several members he is training (the 0..n nearest the Member class). From right to left, a particular member might have a single coach or no coach (the 0..1 nearest the Coach class).
Figure 5-1. Data model for coaches and members (not recommended!)
The problem with the model in Figure 5-1 is that coaches, in all probability, are members of the club. When we implement this model with a Coach table and a Member table, some people will have a row recording their details in each table. For example, Brenda Nolan has a row in the Member table. When she takes up a role as coach, we also would need a row about her in the Coach table. The duplicated information (for example, two phone numbers) is likely to become inconsistent. What is really happening here is not that we have two separate classes of people members and coaches but that we simply have members, some of whom coach other members. This self relationship is shown in Figure 5-2.
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