How to select id with max date group by category in PostgreSQL?

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:

• https://stackoverflow.com/questions/9510509/postgresql-sort-by-datetime-asc-null-first/9511492#9511492

DISTINCT ON is simple and fast. Detailed explanation in this related answer:

• https://stackoverflow.com/questions/3800551/select-first-row-in-each-group-by-group/7630564#7630564

For big tables with many rows per category consider an alternative approach:

• https://stackoverflow.com/questions/25536422/optimize-group-by-query-to-retrieve-latest-row-per-user/25536748#25536748

• https://stackoverflow.com/questions/24244026/optimize-groupwise-maximum-query/24377356#24377356

Try this one:

SELECT t1.* FROM Table1 t1
JOIN 
(
   SELECT category, MAX(date) AS MAXDATE
   FROM Table1
   GROUP BY category
) t2
ON T1.category = t2.category
AND t1.date = t2.MAXDATE

###See this SQLFiddle

Another approach is to use the first_value window function: http://sqlfiddle.com/#!12/7a145/14

SELECT DISTINCT
  first_value("id") OVER (PARTITION BY "category" ORDER BY "date" DESC) 
FROM Table1
ORDER BY 1;

... though I suspect hims056's suggestion will typically perform better where appropriate indexes are present.

A third solution is:

SELECT
  id
FROM (
  SELECT
    id,
    row_number() OVER (PARTITION BY "category" ORDER BY "date" DESC) AS rownum
  FROM Table1
) x
WHERE rownum = 1;