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of the previous query with AND in place of OR will always return an empty answer set A correct statement of Query Q6 using AND is the following: SELECT Ssname FROM Sailors S, Reserves R1, Boats B1, Reserves R2, Boats B2 WHERE Ssid = R1sid AND R1bid = B1bid AND Ssid = R2sid AND R2bid = B2bid AND B1color= red AND B2color = green We can think of R1 and B1 as rows that prove that sailor Ssid has reserved a red boat R2 and B2 similarly prove that the same sailor has reserved a green boat Ssname is not included in the result unless ve such rows S, R1, B1, R2, and B2 are found The previous query is di cult to understand (and also quite ine cient to execute, as it turns out) In particular, the similarity to the previous OR query (Query Q5) is completely lost A better solution for these two queries is to use UNION and INTERSECT The OR query (Query Q5) can be rewritten as follows: SELECT FROM WHERE UNION SELECT FROM WHERE Ssname Sailors S, Reserves R, Boats B Ssid = Rsid AND Rbid = Bbid AND Bcolor = red S2sname Sailors S2, Boats B2, Reserves R2 S2sid = R2sid AND R2bid = B2bid AND B2color = green
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This query says that we want the union of the set of sailors who have reserved red boats and the set of sailors who have reserved green boats In complete symmetry, the AND query (Query Q6) can be rewritten as follows: SELECT Ssname FROM Sailors S, Reserves R, Boats B WHERE Ssid = Rsid AND Rbid = Bbid AND Bcolor = red INTERSECT SELECT S2sname FROM Sailors S2, Boats B2, Reserves R2 WHERE S2sid = R2sid AND R2bid = B2bid AND B2color = green This query actually contains a subtle bug if there are two sailors such as Horatio in our example instances B1, R2, and S3, one of whom has reserved a red boat and the other has reserved a green boat, the name Horatio is returned even though no one individual called Horatio has reserved both a red and a green boat Thus, the query actually computes sailor names such that some sailor with this name has reserved a
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SQL: Queries, Programming, Triggers
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red boat and some sailor with the same name (perhaps a di erent sailor) has reserved a green boat As we observed in 4, the problem arises because we are using sname to identify sailors, and sname is not a key for Sailors! If we select sid instead of sname in the previous query, we would compute the set of sids of sailors who have reserved both red and green boats (To compute the names of such sailors requires a nested query; we will return to this example in Section 544) Our next query illustrates the set-di erence operation in SQL (Q19) Find the sids of all sailors who have reserved red boats but not green boats SELECT FROM WHERE EXCEPT SELECT FROM WHERE Ssid Sailors S, Reserves R, Boats B Ssid = Rsid AND Rbid = Bbid AND Bcolor = red S2sid Sailors S2, Reserves R2, Boats B2 S2sid = R2sid AND R2bid = B2bid AND B2color = green
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Sailors 22, 64, and 31 have reserved red boats Sailors 22, 74, and 31 have reserved green boats Thus, the answer contains just the sid 64 Indeed, since the Reserves relation contains sid information, there is no need to look at the Sailors relation, and we can use the following simpler query: SELECT FROM WHERE EXCEPT SELECT FROM WHERE Rsid Boats B, Reserves R Rbid = Bbid AND Bcolor = red R2sid Boats B2, Reserves R2 R2bid = B2bid AND B2color = green
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Note that UNION, INTERSECT, and EXCEPT can be used on any two tables that are union-compatible, that is, have the same number of columns and the columns, taken in order, have the same types For example, we can write the following query: (Q20) Find all sids of sailors who have a rating of 10 or have reserved boat 104 SELECT Ssid FROM Sailors S WHERE Srating = 10
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