Thread: pgsql aggregate: conditional max
Hi, I need a special aggregation function. For instance, given the following table data: aid | cat | weight ----------+---------+---------a1 | Drama | 1a1 | Romance | 6a1 | Short | 1a1 | Other | 7a2 | Comedy | 1a2 | Drama | 2a3 | Drama | 1a3 | Adult | 2a3 | Comedy | 1a3 | Other | 1 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 Any ideas? Thank you! :) -- All best, Weimao Weimao Ke Indiana University Bloomington School of Library and Information Science http://ella.slis.indiana.edu/~wke
Weimao Ke wrote: > Hi, > > I need a special aggregation function. For instance, given the > following table data: > > aid | cat | weight > ----------+---------+--------- > a1 | Drama | 1 > a1 | Romance | 6 > a1 | Short | 1 > a1 | Other | 7 > a2 | Comedy | 1 > a2 | Drama | 2 > a3 | Drama | 1 > a3 | Adult | 2 > a3 | Comedy | 1 > a3 | Other | 1 > > 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 > > Any ideas? Thank you! :) > Should be able to do this with a standard max() aggregate. select aid, cat, max(weight) from table group by aid, cat; Jeff
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
On Sun, Mar 12, 2006 at 12:34:57AM -0500, Jeffrey Melloy wrote: > Should be able to do this with a standard max() aggregate. > > select aid, cat, max(weight) > from table > group by aid, cat; That query returns the maximum weight for each (aid, cat) pair. Against the example data it returns the entire table, not the (aid, cat) pair with the max weight for a given aid. -- Michael Fuhr
> Hi, > > I need a special aggregation function. For instance, given the following > table data: > > aid | cat | weight > ----------+---------+--------- > a1 | Drama | 1 > a1 | Romance | 6 > a1 | Short | 1 > a1 | Other | 7 > a2 | Comedy | 1 > a2 | Drama | 2 > a3 | Drama | 1 > a3 | Adult | 2 > a3 | Comedy | 1 > a3 | Other | 1 > > 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 > > Any ideas? Thank you! :) > SELECT aid, cat FROM table, ( SELECT aid, max(weight) as weight FROM table GROUP BY aid) AS tablemaxweight WHEREtable.aid = tablemaxweight.aid AND table.weight = tablemaxweight.aid; There is a limit case you don't specify how to deal with, when two or more categories have the same maximum weight. Thequery I wrote retrieves all the categories that have the maximum weight, but perhaps you just want one per aid. -- Daniel
Michael Fuhr wrote: >On Sun, Mar 12, 2006 at 12:34:57AM -0500, Jeffrey Melloy wrote: > > >>Should be able to do this with a standard max() aggregate. >> >>select aid, cat, max(weight) >>from table >>group by aid, cat; >> >> > >That query returns the maximum weight for each (aid, cat) pair. >Against the example data it returns the entire table, not the >(aid, cat) pair with the max weight for a given aid. > > > Michael is right. This query does not solve the problem... -- All best, Weimao Weimao Ke Indiana University Bloomington School of Library and Information Science http://ella.slis.indiana.edu/~wke
Michael Fuhr wrote: >SELECT DISTINCT ON (aid) aid, cat >FROM tablename >ORDER BY aid, weight DESC, cat; > > Good pointer. I think this will solve my problem. :) >SELECT aid, cat >FROM tablename AS t >JOIN (SELECT aid, max(weight) AS weight > FROM tablename > GROUP BY aid) AS s USING (aid, weight); > > This query will return duplicates if there are multiple categories (for one aid) with the same max weight. Yet, I should be able to remove the duplicates somehow...:) I really appreciate your help! -- All best, Weimao Weimao Ke Indiana University Bloomington School of Library and Information Science http://ella.slis.indiana.edu/~wke
Daniel CAUNE wrote: > SELECT aid, cat > > FROM table, ( > SELECT aid, max(weight) as weight > FROM table > GROUP BY aid) AS tablemaxweight > WHERE table.aid = tablemaxweight.aid > AND table.weight = tablemaxweight.aid; > >There is a limit case you don't specify how to deal with, when two or more categories have the same maximum weight. Thequery I wrote retrieves all the categories that have the maximum weight, but perhaps you just want one per aid. > > Yes, this will introduce duplicates. Yet it is not too difficult to select only one for each aid from the results. Thank you! Weimao >-- >Daniel > > > > -- All best, Weimao Weimao Ke Indiana University Bloomington School of Library and Information Science http://ella.slis.indiana.edu/~wke
Hi, Weimao Ke, Weimao Ke wrote: >> SELECT aid, cat >> FROM tablename AS t >> JOIN (SELECT aid, max(weight) AS weight >> FROM tablename >> GROUP BY aid) AS s USING (aid, weight); >> > This query will return duplicates if there are multiple categories (for > one aid) with the same max weight. Yet, I should be able to remove the > duplicates somehow...:) Try SELECT DISTINCT aid, cat FROM tablename AS t JOIN (SELECT aid, max(weight) AS weight FROM tablename GROUP BY aid) AS s USING (aid, weight); HTH, Markus -- Markus Schaber | Logical Tracking&Tracing International AG Dipl. Inf. | Software Development GIS Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org