Question

Possible Duplicate:
SQL: Select first row in each GROUP BY group?

Two SQL tables. One contestant has many entries:

Contestants     Entries
Id   Name       Id  Contestant_Id  Score
--   ----       --  -------------  -----
1    Fred       1   3              100
2    Mary       2   3              22
3    Irving     3   1              888
4    Grizelda   4   4              123
                5   1              19
                6   3              50

Low score wins. Need to retrieve current best scores of all contestants ordered by score:

Best Entries Report
Name     Entry_Id  Score
----     --------  -----
Fred     5         19
Irving   2         22
Grizelda 4         123

I can certainly get this done with many queries. My question is whether there’s a way to get the result with one, efficient SQL query. I can almost see how to do it with GROUP BY, but not quite.

In case it’s relevant, the environment is Rails ActiveRecord and PostgreSQL.

Answer 1

Here is specific postgresql way of doing this:

SELECT DISTINCT ON (c.id) c.name, e.id, e.score
FROM Contestants c
JOIN Entries e ON c.id = e.Contestant_id
ORDER BY c.id, e.score

Details about DISTINCT ON are here.

My SQLFiddle with example.

UPD To order the results by score:

SELECT *
FROM (SELECT DISTINCT ON (c.id) c.name, e.id, e.score
      FROM Contestants c
      JOIN Entries e ON c.id = e.Contestant_id
      ORDER BY c.id, e.score) t
ORDER BY score