For an example, I would like to select id with max date group by category, the result is: 7, 2, 6
id category date
1 a 2013-01-01
2 b 2013-01-03
3 c 2013-01-02
4 a 2013-01-02
5 b 2013-01-02
6 c 2013-01-03
7 a 2013-01-03
8 b 2013-01-01
9 c 2013-01-01
May I know how to do this in PostgreSQL?
This is a perfect use-case for DISTINCT ON
- a Postgres specific extension of the standard DISTINCT
:
SELECT DISTINCT ON (category)
id -- , category, date -- any other column (expression) from the same row
FROM tbl
ORDER BY category, date DESC;
Careful with descending sort order. If the column can be NULL, you may want to add NULLS LAST
:
DISTINCT ON
is simple and fast. Detailed explanation in this related answer:
For big tables with many rows per category
consider an alternative approach: