Question

I have 3 tables:

• questions
• answers
• qa_assoc which standing for question id<->answer id relationship

What I’m trying to do is following: Select all questions, if any question has answer select answer too.

SELECT q.id, q.content, a.id, a.content, a.dt 
        FROM questions q 
        JOIN qa_assoc qaa ON qaa.qid=q.id
        JOIN answers a ON a.id=qaa.aid
        WHERE q.course_id=? AND q.lesson_id=? AND a.user_id=?

But this sql only selects the questions which have an answer. How can I achieve my idea? any suggestions? thx in advance

Answer 1

You can’t have a in your WHERE if you want rows without a, moved it to the ON part

SELECT q.id, q.content, a.id, a.content, a.dt 
FROM questions q 
LEFT JOIN qa_assoc qaa ON (qaa.qid=q.id)
LEFT JOIN answers a ON (a.id=qaa.aid  AND a.user_id=?)
WHERE q.course_id=? AND q.lesson_id=?