I have a table with a large number of records in them. The table has 10-12 fields, two of which are significant. One is the GroupID. There are many GroupIDs, which may or may not be shared amongst a few records each. The second is rating, which ranges from 0 to 100. What I want to do is get the entire row for each row which has the highest rating of its group. I've been having some trouble getting this to work. I tried using Group By on GroupID, and selecting Max(rating) but I can't seem to tell Group By to make the other fields it returns from the same row as where the Max(rating) came from. I've also had some trouble doing it via SubQueries becuase I'm using MySQL which doesn't explicitly support them. Any ideas? Thanks.
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nothingman,
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by: kelfinkPosted on 2002-06-06 at 09:50:19ID: 7059820
Try using a LEFT JOIN, with a forced null comparison:
create table ratings ( groupid int, rating int, data varchar(30) );
insert into ratings values ( 1, 10, 'some row');
insert into ratings values ( 1, 20, 'some other row');
insert into ratings values ( 1, 30, 'the right row for 1');
insert into ratings values ( 2, 10, 'some row');
insert into ratings values ( 2, 20, 'the right row for 2');
insert into ratings values ( 3, 30, 'the right row for 3');
select r.*
from ratings r left join ratings higher
on r.groupid = higher.groupid and r.rating < higher.rating
where higher.groupid is null;
+---------+--------+------
| groupid | rating | data |
+---------+--------+------
| 1 | 30 | the right row for 1 |
| 2 | 20 | the right row for 2 |
| 3 | 30 | the right row for 3 |
+---------+--------+------
3 rows in set (0.00 sec)