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For case x = y: You have to have atleast two unique sets of X and Y values present in the "Functions" table to list one of these sets in your answer.
If you only one set of X and Y with X = Y, although mathematically it'd form a symmetric relation but there is no other set here with which this current set is forming a pair.
Second SELECT query is basically finding all values where X = Y and checking that if there is at least one other set which has same value and if there is, then only we consider it as a pair, otherwise discard it for the purpose of this question.
Symmetric Pairs
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For case x = y: You have to have atleast two unique sets of X and Y values present in the "Functions" table to list one of these sets in your answer.
If you only one set of X and Y with X = Y, although mathematically it'd form a symmetric relation but there is no other set here with which this current set is forming a pair.
Second SELECT query is basically finding all values where X = Y and checking that if there is at least one other set which has same value and if there is, then only we consider it as a pair, otherwise discard it for the purpose of this question.
I hope this made it more clear.