SQL GROUP BY and HAVING Explained: Beginner to Interview Guide
WHERE, GROUP BY, HAVING – Quick Guide Core ideas WHERE filters rows. GROUP BY makes groups of rows. HAVING filters groups (after grouping). 👉 Rule that never changes:Every column in SELECT must be either: in GROUP BY, or an aggregate (SUM, COUNT, AVG, MAX, MIN). Execution order (simplified): FROM / JOIN → WHERE → GROUP BY → HAVING → SELECT → ORDER BY (→ LIMIT/OFFSET if supported) WHERE vs HAVING WHERE HAVING Filters rows Filters groups Used before GROUP BY Used after GROUP BY Cannot use aggregates Can use aggregates WHERE filters rows (pre-aggregation). HAVING filters groups (post-aggregation). Prefer WHERE when possible (filters earlier, usually faster); use HAVING when you need to filter on aggregates. Example using both together Goal: Customers whose individual order is ≥ 100 and total spent > 250. ...