Nested Queries with Correlation

• EXISTS is another set comparison operator, like IN.

• If UNIQUE is used, and * is replaced by R.bid, finds sailors with at most one reservation for boat #103.

  • UNIQUE checks for duplicate tuples; * denotes all attributes. Why do we have to replace * by R.bid?

• Subquery must be recomputed for each Sailors tuple.

  • Think of subquery as a function call that runs a query!

SELECT S.sname

FROM Sailors S

WHERE EXISTS (SELECT *

FROM Reserves R

WHERE R.bid=103 AND S.sid=R.sid)

Find names of sailors who’ve reserved boat #103:

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