08-421-sql1-correlated_v01.pptx - Exists Operator • •...

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1 Exists Operator EXISTS ( relation) is true iff the relation is non-empty Find names of skaters who have participated in competition 101 SELECT S.sname FROM Skaters S WHERE EXISTS (SELECT * FROM Participated P WHERE P.cid = 101 AND P.sid = S.sid) Scoping rule: to refer to outer Skater in the inner subquery, we need to give a range variable to the outer relation. A subquery that refers to values from a surrounding query is called a correlated subquery. Since the inner query depends on the row of the outer query it must be reevaluated for each row in the outer query
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Correlated Query si d snam e ratin g age 28 yuppy 9 15 31 debby 7 10 22
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