First three Groups with Highest Marks should have specific points

Question

my table

+------+-------+---------+-------+--------+
| Name | Group1| Section | Marks | Points |
+------+-------+---------+-------+--------+
| S1   | G1             


        

Answer 1

That was challenging.

To solve it I used several methods:

1. A CASE statement to convert the group's place in the top 3 into points.
2. Rows numbering with a variable.
3. INNER AND LEFT JOIN to merge the results together.

The following query was tested on your fiddle and works:

SELECT t1.`id`,  t1.`name`,  t1.`group1`,
       t1.`section`, t1.`MARKS`, `t_group_points`.`points`

FROM   `students` t1

#--- Join  groups' points to the students
LEFT JOIN (
    (
        #---- Join all groups and give points to top 3 avg's groups ----
        SELECT `t4`.`group1`, `t_points`.`points`
        FROM   (SELECT   `t3`.`group1`, AVG(`t3`.`marks`) AS `avg`
                FROM     `students` `t3`
                WHERE    (`t3`.`section` = 'class1') AND
                         (`t3`.`group1` IS NOT NULL)
                GROUP BY `t3`.`group1`) `t4`

        INNER JOIN (
              #---------- Select top 3 avarages ----------
              (SELECT `top`.`avg`,
                      #-- Convert row number to points ---
                      CASE @curRow := @curRow + 1  
                           WHEN '1' THEN 5
                           WHEN '2' THEN 3
                           WHEN '3' THEN 1
                           ELSE NULL END 'points'

              FROM (SELECT DISTINCT `t_avg`.`avg`
                    FROM   (SELECT   `t2`.`group1`, AVG(`t2`.`marks`) AS `avg`
                            FROM     `students` `t2`
                            WHERE    (`t2`.`section` = 'class1') AND
                                     (`t2`.`group1` IS NOT NULL)
                            GROUP BY `group1`) `t_avg`
                    ORDER BY `avg` DESC
                    LIMIT 0, 3) `top`, (SELECT @curRow:=0) r
              ) AS `t_points`)
         ON (`t_points`.`avg` = `t4`.`avg`)      
    ) AS `t_group_points`)
ON (`t_group_points`.`group1` = `t1`.`group1`)