SQL Topic
Set Operations
UNION, UNION ALL, INTERSECT, and EXCEPT combine results of separate queries. UNION ALL is the tool for stacking rows from two sides — like counting friend relationships in both directions.
When is Set Operations used?
Combining two result sets, counting relationships that exist in either direction, de-duplicating across tables.
Core syntax
SELECT requester_id AS id FROM request_accepted UNION ALL SELECT accepter_id FROM request_accepted.Common mistakes
- Using UNION (dedupes) when UNION ALL (keeps duplicates) is needed for counting.
- Mismatched column counts or types between branches.
- Forgetting ORDER BY must apply to the whole combined result.
Practice Set Operations questions
Work through them in order — each question builds on the last.