Classements et ranking

en sql 
SET @rank := 0;
SELECT nom_ville,CODGEO,nbr_habitants,@rank := @rank + 1 AS rank 
FROM cm_datas 
ORDER BY cast(`nbr_habitants` as decimal) DESC

en php
mysql_query("SET @rank:=0;");
$sql="
SELECT nom_ville,CODGEO,nbr_habitants,@rank := @rank + 1 AS rank 
FROM cm_datas 
ORDER BY cast(nbr_habitants as decimal) DESC
";