Question

I have this table:

+--------+---------+---------+
|subject | student | remark  |
+--------+---------+---------+
| sub1   | stud1   | Pass    |
+--------+---------+---------+
| sub2   | stud1   | Pass    |
+--------+---------+---------+

Subjects sub1 and sub2 are prerequisites to another subject. I want to extract the student’s remark from those two prerequisite subject to see if he is qualified to enroll.

I currently have:

SELECT enrollees.student, students_db.lastname, students_db.firstname FROM enrollees, students_db WHERE 
(remark = 'Pass' AND subj = 'sub1' AND subj = 'sub2')
AND enrollees.student = students_db.student_id

This query yields no result.

Answer 1

It’s because in your WHERE condition you’re wanting subj='sub1' AND subj='sub2' simultaneously, which can never be possible.

One way to do it is to JOIN the table with sub and mark (enrolees?). This is because the student has to pass BOTH sub1 and sub2 (as opposed to passing AT LEAST ONE, in which case you can use and subj IN ('sub1','sub2'):

SELECT e1.student, sdb.lastname, sdb.firstname 
FROM students_db sdb
JOIN enrollees e1 ON e1.student=sdb.student_id
JOIN enrollees e2 ON e2.student=sdb.student_id
WHERE 
e1.sub = 'sub1' AND e1.remark = 'Pass'
AND
e2.sub = 'sub2' AND e2.remark = 'Pass'

This ensures that the student passes both sub1 and sub2.

However, it’s a bit sucky – if you have 5 prerequisites, all necessary, you have to JOIN in enrollees 5 times.

One way you can get around this, is to SUM up the number of passes they got. Then you don’t have to JOIN:

SELECT e.student, sdb.lastname, sdb.firstname,
FROM students_db sdb
JOIN enrollees e ON e.student=sdb.student_id
WHERE e.remark='Pass' AND e.sub IN ('sub1','sub2') -- list of subjects here
GROUP BY e.student
HAVING SUM(IF(e.remark='Pass',1,0))=2  -- number of subjects in the list here

This counts up how many of the required subjects (in this case, ‘sub1’ and ‘sub2’) the student has passed, and makes sure that is equal to 2 (since they have to have passed all subjects).

This has the advantage of you being able to put in as many prerequisite subjects as you want (provided all are required prerequisites) — you just have to list them in the WHERE and change the total in the HAVING. The number of joins does not grow with the number of prerequisites.