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CHAPTER 7 SE T OPERA TION S
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Listing 7-22. Finding the Names That Are in ClubA But Not ClubB SELECT FamilyName, Name FROM ClubA EXCEPT SELECT LastName, FirstName FROM ClubB
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Uses of Difference
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Whenever you have a query that has the word not, you should consider the possibility that the difference operator will be useful. For example, how do we find members who have not entered tournament 25 Recall from 5 why the query in Listing 7-23 does not give us the correct rows from the Entry table.
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Listing 7-23. Members Who Have Not Entered Tournament 25 (Incorrect) SELECT MemberID FROM Entry WHERE TourID <> 25
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Listing 7-23 selects all the rows in the Entry table that are not for tournament 25. Looking at Figure 7-15, we can see that the query would return the row marked e2 for member 228 entering tournament 36. However, in the row above, we see that member 228 has also entered tournament 25. This is not what we might naively have expected from Listing 7-23 (before reading 5, of course!). What we need is a list of all members, a list of all the members entering tournament 25, and then to take the difference between them, as in Figure 7-17. The set of rows on the left in Figure 7-17 all member IDs is retrieved by projecting the MemberID column from the Member table. The rows in the middle of Figure 7-17 IDs of all members entering tournament 25 are found by selecting the rows from the Entry table for tournament 25 and then projecting just the MemberID column. We then need the difference between these two sets of rows to find the IDs of members who have not entered tournament 25. The complete algebra and SQL expressions to retrieve the IDs of members who have not entered tournament 25 are in Listings 7-24 and 7-25.
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Listing 7-24. Algebra to Retrieve IDs of Members Who Have Not Entered Tournament 25
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Listing 7-25. SQL to Retrieve IDs of Members Who Have Not Entered Tournament 25 SELECT MemberID FROM Member EXCEPT SELECT MemberID FROM Entry WHERE TourID = 25
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C HA PTER 7 S ET O PE RAT IO NS
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A IDs of all members
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B IDs of members entering tournament 25
A B IDs of members who have not entered tournament 25
Figure 7-17. Using the difference operator to find members who have not entered tournament 25
As with intersection and union operations, it is important that we project the appropriate columns before we use the difference operator. In Figure 7-17, we have retrieved the IDs from the Member and Entry tables. If we want to include the names of the members, we can use one of the methods explained in the Uses of Intersection section earlier in this chapter. However, in this difference example, we already had the names of the members in the Member table before we removed them to get the set of rows on the left side of Figure 7-17. It seems a bit perverse to remove the names and then put them back later. What is important is that the two sets of rows involved in the difference are union compatible; that is, the corresponding columns must have the same domains. Either both sets have just IDs or both sets have IDs and names. In the operation on the left side of Figure 7-17, we took the first option and removed the names from Member. We could have left the names in the Member table and added the names to the rows in the middle of Figure 7-17 by joining the Entry and Member tables, as shown in Figure 7-18. We can then take the difference between these two sets of rows.
CHAPTER 7 SE T OPERA TION S
a) IDs and names from Member
MemberID,LastName,FirstNameMember
b) Entry table first joined with Member table to include names of members entering tournament 25 (Member
MemberID,LastName,FirstName TourID=25 MemberID=MemberIDEntry)
Figure 7-18. Including names of members in both sets of rows before taking the difference
The SQL equivalent of the operations shown in Figure 7-18 is given in Listing 7-26.
Listing 7-26. Including the Names Before Taking the Difference SELECT MemberID, LastName, FirstName FROM Member EXCEPT SELECT m.MemberID, m.LastName, m.FirstName FROM Entry e inner join Member m on e.MemberID = m.MemberID WHERE TourID = 25
Another use for the difference operation is in checking or validating data from different sources. By using the difference operation, and with appropriate projecting of columns, you can check if any instances have been added or deleted during translation.
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