Group and Rank Rows in Mysql - php

Question

I'm trying to build a ranking system in a mysql database.
I've found several tutorials on ranking and items here on StackOverflow about ranking individual rows against each other.
However, my issue is that I need to group rows by a user id column, add up the values to a second column grouped by user id, then rank them against other groups of a different user id.
Here's an example of the table I'm using:
user_id km_skied date_entered
1 34 2010-08-19
3 2 2010-08-23
1 3 2010-08-13
4 23 2010-08-01
3 5 2010-08-02
The result printout would be by rank:
Skier Rank:
Rank User ID Total KM
1 1 37
2 4 23
3 3 7
Also, I was wondering how I find the rank for a specific user. Meaning, if I know what the user id is, can I give them just their rank? Like say
"Your Rank: 2 of 345"
That is the second part of this.
Anyone know how to do that?
Thanks!
Troy

Answer 1

Your query should look something like this. Add the ranking logic to the outer loop.
select * from
(select user_id, sum(km_skied) as km from ski group by user_id) x
order by x.km desc;
Don't know if it's an option, but you can use a temporary table for rankings as follows:
create temporary table ranks (rank int primary key auto_increment, user_id int, km int);
insert into ranks (user_id, km)
select user_id, km from (
select user_id, sum(km_skied) as km from ski group by user_id
) x order by x.km desc;
This gives you what you want:
mysql> select * from ranks;
+------+---------+------+
| rank | user_id | km |
+------+---------+------+
| 1 | 1 | 37 |
| 2 | 4 | 23 |
| 3 | 3 | 7 |
+------+---------+------+
3 rows in set (0.00 sec)
One downside to this approach is that skiers who are tied won't get the same rank.

Answer 2

Do the grouping in subquery and ranking of the results (using any of the methods you've found before) in outer query.

Answer 3

Thanks for your help guys.
I was able to come up with an answer based on the following Query:
$totalQuery = "SELECT SUM(track_length) as usertracklength, username, MAX(track_create_time) as lasttrack, count(DISTINCT track_create_time) as totaldays FROM user_tracks GROUP BY username ORDER BY usertracklength DESC";
$totalResult = mysql_query($totalQuery);
$rankResult = mysql_query($totalQuery);
$totalNumEntries = mysql_num_rows($totalResult);
Then Ouputting that to an array
// rank position array
$rankArray = array();
while ($row1 = mysql_fetch_array($rankResult)) {
$rankArray[] = $row1['username'];
}
Then finding position of that username in the array by using a foreach in php
foreach ($rankArray as $rank => $user) {
if ($user == $username) {
$yourRank = $rank+1;
}
}
It's the long way around, but I suppose it works for what I'm going for.
Was kind of hoping to get it done within the mysql query for efficiency.
Thanks!

Answer 4

You could try grouping to sum the Km as a first query, then follow it by a correlated subquery to find the ranks. For instance, if your values are stored in a table called "test", sum the Km values into a table called testtbl and then do the ranking.
mysql> select * from test;
+------+--------+------+
| Id | km_run | name |
+------+--------+------+
| 1 | 34 | a |
| 3 | 2 | c |
| 1 | 3 | a |
| 4 | 23 | d |
| 3 | 5 | c |
+------+--------+------+
5 rows in set (0.00 sec)
mysql> create table testtbl as
(select Id, sum(km_run) as tot
from test
group by Id);
Query OK, 3 rows affected (0.04 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from testtbl;
+------+------+
| Id | tot |
+------+------+
| 1 | 37 |
| 3 | 7 |
| 4 | 23 |
+------+------+
3 rows in set (0.00 sec)
mysql> select t1.Id,t1.tot,
((select count(distinct t2.tot) from testtbl t2 where t1.tot < t2.tot)+1) as Rk from testtbl t1
order by Rk;
+------+------+------+
| Id | tot | Rk |
+------+------+------+
| 1 | 37 | 1 |
| 4 | 23 | 2 |
| 3 | 7 | 3 |
+------+------+------+
3 rows in set (0.00 sec)