26.4. Equality and Proper Subsets
At one point, when SQL was still in the laboratory at IBM, there was a CONTAINS operator that would tell you if one table was a subset of another. It disappeared in later versions of the language and no vendor picked it up. Set equality was never part of SQL as an operator, so you would have to have used the two expressions ((A CONTAINS B) AND (B CONTAINS A)) to find out.
Today, you can use the methods shown in the section on Relational Division to determine containment or equality. However, Itzik Ben-Gan came up with a novel approach for finding containment and equality that is worth a mention.
SELECT SUM(DISTINCT match_col) FROM (SELECT CASE WHEN S1.col IN (SELECT S2.col FROM S2) THEN 1 ELSE -1 END FROM S1) ...
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