On Sun, Mar 12, 2006 at 12:09:48AM -0500, Weimao Ke wrote: > I want to group by "aid" and choose the category (i.e., "cat") with the > largest "weight": > > aid | max_weighted_cat > ----+--------------------- > a1 | Other > a2 | Drama > a3 | Adult
PostgreSQL has a non-standard DISTINCT ON clause that would work. See the weather_reports example in the documentation for SELECT: http://www.postgresql.org/docs/8.1/interactive/sql-select.html Try this query against your example data: SELECT DISTINCT ON (aid) aid, cat FROM tablename ORDER BY aid, weight DESC, cat; If multiple rows for a given aid match that aid's max weight then the above query will return the first matching row according to the given sort order. Some people object to DISTINCT ON because it's non-deterministic if you don't order by enough columns. Here's something more standard; it'll return all rows that match a given aid's max weight: SELECT aid, cat FROM tablename AS t JOIN (SELECT aid, max(weight) AS weight FROM tablename GROUP BY aid) AS s USING (aid, weight); -- Michael Fuhr ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
