UNION, INTERSECT and EXCEPT

UNION ALL stacks two results on top of each other; both sides need the same number of columns in the same order. UNION does the same but removes duplicates, which costs a sort.

INTERSECT keeps rows present in both results and EXCEPT keeps rows present in the first but not the second — a quick way to diff two populations.

Prefer UNION ALL unless you actually need de-duplication — it is cheaper, and it keeps legitimately repeated rows.