Question

I have 2 tables:

• TOPICS (id, title)
• ANSWERS (id, id_topic, id_user, answer)

…and i want to do a select to detect all the questions belonging to a user in one select.

I tried to do a join, but that doesn’t work since if a user answered a topic twice, it will return 2 rows with the same topic. I tried to do a SELECT DISTINCT, but that fails also.

Answer 1

Try:

SELECT id, topic 
  FROM topics 
 WHERE id IN (SELECT id_topic 
                FROM answers 
                WHERE id_user = <USERID>)

…or:

SELECT id, topic 
  FROM topics 
 WHERE EXISTS (SELECT * 
                 FROM answers 
                WHERE id_user = topics.id)

…and test for performance according to your indexing regimen. Recent versions of MySQL only.