Which set operators are order-sensitive?

Question

Accepted Answer

UNION and INTERSECT produce the same result regardless of which side each query is on, because 'all rows from either' and 'rows in both' do not depend on order. EXCEPT is directional: `A EXCEPT B` removes B from A, while `B EXCEPT A` removes A from B - these are different operations. A practical consequence is that accidentally swapping the operands of EXCEPT is a silent logic bug - the query runs without error but returns wrong data.