PHP MySQLi Ranking Position

I'm developing a scoreboard of sorts. The table structure is ID, UID, points with UID being linked to a users account.

Now, I have this working somewhat, but I need one specific thing for this query to be pretty much perfect. To pick a user based on rank.

I'll show you my the SQL.

SELECT *, @rownum := @rownum + 1 AS `rank` FROM 
            (SELECT * FROM `points_table` `p` 
             ORDER BY `p`.`points` DESC 
             LIMIT 1) 
        `user_rank`, 
        (SELECT @rownum := 0) `r`, `accounts_table` `a`, `points_table` `p`
        WHERE `a`.`ID` = `p`.`UID`

It's simple to have it pick people out by UID, but that's no good. I need this to pull the user by their rank (which is a, um, fake field ^_^' created on the fly). This is a bit too complex for me as my SQL knowledge is enough for simple queries, I have never delved into alias' or nested queries, so you'll have to explain fairly simply so I can get a grasp.

Solution

I think there is two problems here. From what I can gather you want to do a join on two tables, order them by points and then return the nth record. I've put together an UNTESTED query. The inner query does a join on the two tables and the outer query specifies that only a specific row is returned.

This example returns the 4th row.

SELECT * FROM
(SELECT *, @rownum := @rownum + 1 AS rank
FROM `points_table` `p` 
JOIN `accounts_table` `a` ON a.ID = p.UID,
(SELECT @rownum:=0) r
ORDER BY `p`.`points` DESC) mytable
WHERE rank = 4

Hopefully this works for you!

I've made a change to the answer which should hopefully resolve that problem. Incidentally, whether you use a php or mysql to get the rank, you are still putting a heavy strain on resources. Before mysql can calculate the rank it must create a table of every user and then order them. So you are just moving the work from one area to another. As the number of users increases, so too will the query execution time regardless of your solution. MySQL will probably take slightly longer to perform calculations which is why PHP is probably a more ideal solution. But I also know from experience, that sometimes extraneous details prevent you from having a completely elegant solution. Hope the altered code works.