Lesson 5 of 13 · 16 min
HAVING: filtering groups
By the end of this lesson you can filter grouped results with HAVING, combine it with WHERE in one query, and explain the order in which PostgreSQL evaluates the clauses.
WHERE filters rows. It looks at one row at a time and decides whether that row goes into the groups. It cannot use COUNT(*) or SUM(amount), because while WHERE runs, no groups exist yet.
HAVING filters groups. It runs after GROUP BY has built the groups and their aggregates, and keeps only the groups whose condition is true. A group that fails HAVING disappears from the result as a whole.
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