How to avoid repetitions of columns in MySQL subquery? - mysql

Question

i am retrieving the result from above 4 table using following query
SELECT
(SELECT SUM(CASE when c.Training_Id=1 then 1 else 0 end)
FROM courses c
INNER JOIN enrolled_students es
ON c.Course_Id = es.Course_Id
) STEM,
(SELECT SUM(CASE when c.Training_Id=2 then 1 else 0 end)
FROM courses c
INNER JOIN enrolled_students es ON c.Course_Id = es.Course_Id
) MA,
c.* FROM campus c;
The problem with this query is, two(2) students are in STEM and one(1) Student in MA against Campus_Id 3, but its repeating records against all campuses. i want if campus has no students than there should be '0' Zero.

Answer 1

You need to filter your subselects by Campus_Id. But first you have to use distinct table aliases. Change your last line to ca.* FROM campus ca. Then you can use a where clause in your subselects (WHERE c.Campus_Id = ca.Campus_Id).
SELECT
(SELECT SUM(CASE when c.Training_Id=1 then 1 else 0 end)
FROM courses c
INNER JOIN enrolled_students es
ON c.Course_Id = es.Course_Id
WHERE c.Campus_Id = ca.Campus_Id -- line added
) STEM,
(SELECT SUM(CASE when c.Training_Id=2 then 1 else 0 end)
FROM courses c
INNER JOIN enrolled_students es ON c.Course_Id = es.Course_Id
WHERE c.Campus_Id = ca.Campus_Id -- line added
) MA,
ca.* FROM campus ca; -- line changed
This should solve your problem.
To improve the performance you can also filter your subselects by Training_Id. In the first subselect you only need the rows with Training_Id=1. So you can change your where clause to:
WHERE c.Campus_Id = ca.Campus_Id
AND c.Training_Id = 1
Doing that you can also use COUNT instead of SUM. So your subselect would look like:
SELECT COUNT(1)
FROM courses c
INNER JOIN enrolled_students es ON c.Course_Id = es.Course_Id
WHERE c.Campus_Id = ca.Campus_Id AND c.Training_Id = 1
To prevent code duplication (your subselects are almost equal) you can join all needed tables and group by Campus_Id:
select
COUNT(co.Training_Id=1 OR NULL) STEM,
COUNT(co.Training_Id=2 OR NULL) MA,
ca.Campus_Id
from campus ca
left join courses co on co.Campus_Id = ca.Campus_Id
left join enrolled_students es on es.Course_Id = co.Course_Id
where co.Training_Id in (1, 2)
group by ca.Campus_Id