Merge two similar aggregate functions in select statement - mysql

Question

Here's a SQL statement:
SELECT DISTINCT `class`, `student_id` , `student_name`,
(
SELECT SUM( `credits` )
FROM `stumgr_scores` B
JOIN `stumgr_courses` USING ( `course_id` )
WHERE `year` =2012 AND A.`student_id` = B.`student_id`
) AS `total_credits`,
(
SELECT SUM( `credits` * `final_score` )
FROM `stumgr_scores` C
JOIN `stumgr_courses` USING ( `course_id` )
WHERE `year` =2012 AND A.`student_id` = C.`student_id`
) AS `total_scores`
FROM `stumgr_scores` A
NATURAL JOIN `stumgr_students`
WHERE `year` =2012 AND `grade` =2011
You may discover that these two select statement which use aggregate functions are similar. So, I want to merge them into one as following:
SELECT DISTINCT `class`, `student_id` , `student_name`,
(
SELECT
SUM( `credits` ) AS `total_credits`,
SUM( `credits` * `final_score` ) AS `total_scores`
FROM `stumgr_scores` B
JOIN `stumgr_courses` USING ( `course_id` )
WHERE `year` =2012 AND A.`student_id` = B.`student_id`
) AS `something`
FROM `stumgr_scores` A
NATURAL JOIN `stumgr_students`
WHERE `year` =2012 AND `grade` =2011
Of course, the SQL statement above doesn't work and I don't know what to do.
Besides, the query is very slow because of large data, do you have any suggestions? Thanks a lot.

Answer 1

I have had to guess at your table structure slightly, but you should be able to simplify this query massively by using JOINs rather than correlated subqueries:
SELECT st.student_id,
st.student_name,
c.class,
SUM(sc.credits) AS total_credits,
SUM(sc.credits * sc.final_score) AS total_scores
FROM stumgr_students st
INNER JOIN stumgr_scores sc
ON sc.student_id = st.student_id
INNER JOIN stumgr_courses c
ON c.course_id = st.course_id
GROUP BY st.student_id, st.student_name, c.class;