SELECT 'foo' WHERE bar = baz FROM quux UNION ALL SELECT 'bar' WHERE bar <> baz FROM quux I would understand returning an error in that case if bar or baz w

SELECT 'foo' WHERE bar = baz FROM quux UNION ALL SELECT 'bar' WHERE bar <> baz FROM quux I would understand returning an error in that case if bar or baz were unknown. The truthiness of bar = baz is unknown, so that's an error. But that's not what SQL Servers do. They're all pedantic about how NULL means unknown right up until the rubber hits the road of the WHERE clause, when suddenly NULL means false. Which means that they're not saying that foo = bar is unknown, they're instead making a definitive assertion that both foo = bar is false and foo <> bar is false. And that's disastrously wrong.