US Patent 7676450 Null aware anti-join

Techniques for performing a “null-aware” anti-join operation are described. Unnesting using anti-join of NOT IN/ALL subquery uses null-aware anti-join operation, resulting in a rewritten query that, when computed, produces results consistent with the NULL semantics of NOT IN/ALL subquery. The semantics of the “null-aware” anti-join operation allow the query having the NOT IN/ALL subquery to be rewritten even though a no-NULL restriction requirement, for the operands of the anti-join condition in the query, may not be met.