UNION, INTERSECT & EXCEPT

Combine the rows of two queries as sets.

Stack query results

Set operators combine the rows of two SELECTs that have the same columns:

UNION — all rows from both, duplicates removed
UNION ALL — all rows, duplicates kept (faster — use when you know there are none)
INTERSECT — only rows in both
EXCEPT — rows in the first but not the second

SELECT book_id FROM order_items
INTERSECT
SELECT book_id FROM reviews;

Column names come from the first query; the column types must line up.

Ordered ids are 1–7, 9 and 10; reviewed ids are 1, 2 and 4–7. EXCEPT is the ordered-but-never-reviewed answer, INTERSECT is the overlap, and UNION ALL is the only one that keeps duplicates.