Avoiding the (a,b)/(b,a) Duplicate

Concepts covered: sqlSelfJoin

Symmetric pair questions ('find customers in the same region as another customer') are where the duplicate-pair bug ships most often. The naive query returns each pair twice plus every self-pair, inflating the row count by 2N + N². The fix is one inequality, but the choice of which column to use as the tiebreaker has implications for correctness, cardinality, and deduplication semantics. The bug and the standard fix Choosing the column and the direction Two decisions sit underneath the inequality. First: the column. The inequality column must be unique per row. customer_id is the standard choice because it is the table's primary key; order_id, transaction_id, email, anything-with-a-uniqueness-constraint works. Using a non-unique column for the inequality breaks the deduplication: if two cu

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