PostgreSQL: getting ordinal rank (row index? ) eff

Question

You have a table like so:

id dollars dollars_rank points points_rank
1  20      1            35     1
2  18      2            30     3
3  10      3            33     2

I want a query that updates the table's rank columns (dollars_rank and points_rank) to set the rank for the given ID, which is just the row's index for that ID sorted by the relevant column in a descending order. How best to do this in PostgreSQL?

Answer 1

@OMG_Ponies already pointed it out: The window function dense_rank() is what you need - or maybe rank(). The UPDATE could look like this:

Test case:

CREATE TEMP TABLE tbl (
   id int
 , dollars int
 , dollars_rank int
 , points int
 , points_rank int
 );
INSERT INTO tbl VALUES
 (1, 20, 1, 35, 1)
,(2, 18, 2, 30, 3)
,(3, 10, 3, 33, 2)
,(4, 10, 3, 33, 2);  -- I put a dupe row in to demonstrate ties.

UPDATE statement:

UPDATE tbl
SET    dollars_rank = r.d_rnk
     , points_rank  = r.p_rnk
FROM (
    SELECT id
         , dense_rank() OVER (ORDER BY dollars DESC) AS d_rnk
         , dense_rank() OVER (ORDER BY points DESC)  AS p_rnk
    FROM   tbl
    ) r
WHERE tbl.id = r.id

You need PostgreSQL 8.4 or later for window functions.