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Tuesday, June 24, 2008

Exists vs. In

The following two queries produce the same result. Using Exists can be a lot faster since often only an index needs to be checked.
Select Table_Name
From User_Tables
Where Table_Name In (Select Table_Name
From User_Constraints

Select Table_Name
From User_Tables Ut
Where Exists (Select 1
From User_Constraints Uc
Where Ut.Table_Name = Uc.Table_Name
See Oracle documentation for further detail:
Correlated Subquery

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