MySQL subquery unknown column for complex query - mysql

Question

I've got the following sqlfiddle http://sqlfiddle.com/#!2/324628/1
I need to create a query that returns the id and the position (ranking) of each student within his class; the position is sorted in descending order according to the value of their academic average stored inside the academic_averages table.
(e.g. the first from class 1, second from the class 1, and so on... the first from class 2, the second from class 2...)
Here's the query:
SELECT students.id,
(SELECT x.position
FROM (
SELECT t.student_id, t.value, #rownum := #rownum + 1 AS position
FROM (
SELECT aa.student_id, aa.value
FROM academic_averages AS aa
INNER JOIN students AS s ON s.id = aa.student_id
INNER JOIN classes_students AS cs ON cs.student_id = s.id
INNER JOIN classes_academic_years AS cas ON cas.id = cs.class_academic_year_id
INNER JOIN classes_academic_years as cas2 on cas2.class_id = cas.class_id
INNER JOIN classes_students as cs2 on cs2.class_academic_year_id = cas2.id
INNER JOIN students as s2 on s2.id = cs2.student_id
WHERE s2.id = 243
AND cas.academic_year_id = 4
AND aa.academic_year_id = 4
GROUP BY aa.student_id
ORDER BY abs(aa.value) DESC
) t
JOIN (SELECT #rownum := 0) r
) AS x WHERE x.student_id = students.id ) AS ranking_by_class
FROM students
However, since it contains a subquery, I cannot change the WHERE from the inner most query to s2.id = students.id because it throws an error (unknown column).
I've tried using an INNER JOIN instead of subqueries, but no luck so far.
Does anyone have a solution?
Thanks
LE: Performance wise the query must be optimized
LE: Here's the structure of the tables:
academic_averages:
id
student_id
value
academic_year_id
classes_academic_years:
id
class_id
name
grade
academic_year_id
classes_students:
id
class_academic_year_id
student_id
classes:
id
school_id
students:
id
The desired output should be student_id, position.
There seems to be some issues with sql fiddle, meanwhile here's the schema: http://snippi.com/s/db8za8k

Answer 1

SELECT x.id
, x.position
, x.academic_average
FROM (SELECT
s.id
, #rownum := #rownum + 1 position
, av.value academic_average
FROM students s
JOIN classes_students cs ON s.id = cs.student_id
JOIN classes_academic_years cay ON cay.id = cs.class_academic_year_id
JOIN academic_averages av ON av.student_id = s.id
WHERE cay.academic_year_id = 4 -- change these two parameters in
AND av.academic_year_id = 4 -- the subquery for different years
ORDER BY av.value DESC) x,
(SELECT #rownum := 0) y
ORDER BY academic_average DESC
I think the above query should work for you.
I've made the assumption that the academic ranking position is determined in a descending order by academic average.
I don't have access to your dataset, so I've added three extra lines, two to select the students' academic average and one to order the result in descending order according to the academic average. This should help you verify that it works as intended. If you run the query, and it works, it should display records with position starting at 1 and incrementing by 1.
In production I would omit these fragments in order to get the result set you specify:
1. , x.academic_average
2. , av.value academic_average
3. ORDER BY academic_average DESC
Edit following elaboration in comments by OP (students' ranking required by class)
This query should give you students' positions by class. If you want to get rid of some fields, you can wrap the SELECT in another SELECT, or ignore the columns once the dataset is extracted to another language.
SELECT
x.student_id
, x.cay_class_id
, x.academic_average
, if(#classid = x.cay_class_id, #rownum := #rownum + 1, #rownum := 1) position
, #classid := x.cay_class_id
FROM (SELECT
s.id student_id
, cay.class_id cay_class_id
, av.value academic_average
FROM students s
JOIN classes_students cs ON s.id = cs.student_id
JOIN classes_academic_years cay ON cay.id = cs.class_academic_year_id
JOIN academic_averages av ON av.student_id = s.id
WHERE cay.academic_year_id = 4 -- change these two parameters in
AND av.academic_year_id = 4 -- the subquery for different years
ORDER BY cay.class_id, av.value DESC) x,
(SELECT #classid := 0, #rownum := 0) y