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Using IN with a Nested Query
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The real usefulness of the IN keyword is that we can use another SQL statement to generate the values in the set. For example, the reason that someone may have been interested in the set of tournaments (36, 38, 40) might have been because they are the Open tournaments. Thus, we might want to generate a list of Open tournaments and feed that list into the IN clause. Let s look at a specific example of a query feeding into IN. I ve reproduced a few of the columns of the Member table along with the Entry and Tournament tables in Figure 4-1. In Listings 4-1 and 4-2, the Open tournaments were explicitly stated in the query by listing each ID. If a new Open tournament is added to the Tournament table, then the query will need to be changed to include that tournament s ID in the set. However, we can construct another query to retrieve the IDs of all the Open tournaments, as shown in Listing 4-4.
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Listing 4-4. Finding the IDs of All Open Tournaments SELECT t.TourID FROM Tournament t WHERE t.TourType = 'Open'
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CHAPTER 4 NES TE D QUERIES
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(Some columns) Member
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Entry
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Tournament
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Figure 4-1. Member, Entry, and Tournament tables
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We can replace the list of explicit values (36, 38, 40) in Listing 4-2 with the SQL statement (Listing 4-4) that will retrieve the current values for Open tournaments, as shown in Listing 4-5. I ve indented the nested part of the query (sometimes called a subquery) so you can see it more clearly.
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Listing 4-5. A Nested Query to Find All Entries in Open Tournaments SELECT e.MemberID FROM Entry e WHERE e.TourID IN (SELECT t.TourID FROM Tournament t WHERE t.TourType = 'Open')
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You can understand a nested query by reading it from the inside out. The inside SELECT statement retrieves the set of required tournament IDs from the Tournament table, and then the outside SELECT finds us all the entries from the Entry table for tournaments IN that set. To work correctly with the IN keyword, the nested part of the query must return a list of single values.
CHAPTER 4 NES TED QUER IES
Have another look at the tables in Figure 4-1. How else might we have retrieved entries for Open tournaments We did this in the previous chapter using a join. We can join the two tables Entry and Tournament on their common fields TourID. Then select just those rows that are for Open tournaments and retrieve (or project) the MemberID column. The SQL statement is in Listing 4-6.
Listing 4-6. Using a Join to Find All Entries in Open Tournaments SELECT e.MemberID FROM Entry e INNER JOIN Tournament t ON e.TourID = t.TourID WHERE t.TourType = 'Open'
The SQL statements in Listings 4-5 and 4-6 retrieve the same information. As I ve said a number of times, there are often several different ways to write a query in SQL. The more methods you are familiar with, the more likely you will be able to find a way to express a complicated query.
Being Careful with NOT and <>
As well as asking a question such as What are the IDs of members who have entered an Open tournament it is just as likely that we might want to know What are the IDs of members who have NOT entered an Open tournament They sound very similar, but once we start using negatives in our questions, we have to be very careful about what we really mean. In 7, we will investigate such questions using set operations, but to keep this chapter complete, I ll talk about how negatives impact the use of nested queries in particular. Listings 4-5 and 4-6 showed two SQL statements for retrieving member IDs for members who have entered an Open tournament. As a first attempt, novices will often amend these queries slightly by changing = to <> or by changing IN to NOT IN, as in Listing 4-7 and Listing 4-8.
Listing 4-7. Using NOT IN: What Does This Query Retrieve SELECT e.MemberID FROM Entry e WHERE e.TourID NOT IN (SELECT t.TourID FROM Tournament t WHERE t.TourType = 'Open')
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