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:
Exists
Correlated Subquery