SUM for the 5 best models in 5 best brands

As we can see in the solution:

Expression are evaluated in the column order in the select clause.

This is the same for the where clause.  An advice is to test first the expression that get more chance to return false at the head of the where conditions :   

1. SET @BRAND=0;
2. SET @ct=0;
3. SELECT
4.  *
5. FROM
6.  (SELECT
7.    IF(@BRAND<>id_BRAND,@ct:=0 ,@ct:=@ct ) ,
8.    @ct:=@ct+1 AS ct,   
9.    @BRAND:=id_BRAND,
10.    t2.*
11.   FROM
12.    (SELECT
13.      st.ID_BRAND,
14.      st.ID_MODEL,
15.      SUM(COMPTE) AS total,
16.      AVG(totalm)
17.     FROM
18.      STAT st JOIN
19.      (SELECT
20.        ID_BRAND,
21.        SUM(COMPTE) AS totalm
22.       FROM
23.        STAT
24.       GROUP BY
25.        ID_BRAND
26.       ORDER BY
27.        totalm DESC
28.       LIMIT 5
29.      ) as t USING( ID_BRAND)
30.     GROUP BY
31.      st.ID_BRAND,
32.      st.ID_MODEL
33.     ORDER BY
34.      st.ID_BRAND,
35.      total DESC
36.    ) AS t2
37.  ) AS t3
38. WHERE
39.  ct <=5