Question

Left outer joins should return at least one row from the T1 table if it matches the conditions. But what if the left outer join performs a join successfully, then finds that another criterion is not satisfied? Is there a way to get the query to return a row with T1 values and T2 values set to NULL?

Here’s the specific query, in which I’m trying to return a list of candidates, and the user’s support for those candidates IF such support exists.

   SELECT c.id, c.name, s.support  
     FROM candidates c
LEFT JOIN support s on s.candidate_id = c.id
    WHERE c.office_id = 5059 
      AND c.election_id = 92 
      AND (s.user_id = 2 OR s.user_id IS NULL)  --This line seems like the problem
 ORDER BY c.last_name, c.name

The query joins the candidates and support table, but finds that it’s a different user who supported this candidate (user_id=3, say). Then the candidate disappears entirely from the result set.

Answer 1

Can’t you move the condition from the where to the join clause?

LEFT JOIN support s on s.candidate_id=c.id and s.user_id = 2