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    <p>It's because in your <code>WHERE</code> condition you're wanting <code>subj='sub1'</code> AND <code>subj='sub2'</code> simultaneously, which can never be possible.</p> <p>One way to do it is to JOIN the table with <code>sub</code> and <code>mark</code> (<code>enrolees</code>?). This is because the student has to pass <em>BOTH</em> sub1 and sub2 (as opposed to passing AT LEAST ONE, in which case you can use <code>and subj IN ('sub1','sub2')</code>:</p> <pre><code>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' </code></pre> <p>This ensures that the student passes <em>both</em> sub1 and sub2.</p> <p>However, it's a bit sucky - if you have 5 prerequisites, all necessary, you have to <code>JOIN</code> in <code>enrollees</code> 5 times.</p> <p>One way you can get around this, is to SUM up the number of passes they got. Then you don't have to JOIN:</p> <pre><code>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 </code></pre> <p>This counts up how many of the <strong>required subjects</strong> (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).</p> <p>This has the advantage of you being able to put in <em>as many prerequisite subjects as you want</em> (provided all are required prerequisites) -- you just have to list them in the <code>WHERE</code> and change the total in the <code>HAVING</code>. The number of joins does not grow with the number of prerequisites.</p>
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    1. COThat's exactly what I'm trying to do that' why I kept using AND. *Noob Me*. Thank you, sir. Your response was the right one. Thank you.
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