SQL: Join on two tables where on column is null

Question

I am using mysql database. I have two tables bugs and profiles. Bugs table has two columns (assigned_to, qa_contact) pointing to profiles by a many to one relationship. These are the simplified version of my queries.

Firstly, I was trying to do this but it returns duplicate rows where qa_contact is null in the bugs table

select 
  bug_id, 
  desc, 
  dev.assigned_to, 
  qa.qa_contact 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and (qa_contact = qa.userid or qa_contact is null)

Secondly, my new approach is:

select bug_id, desc, dev.assigned_to, qa.qa_contact 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and qa_contact = qa.userid

 UNION

select bug_id, desc, dev.assigned_to, null 
from 
 bugs, 
 profiles as dev, 
 profiles as qa
where 
  assigned_to = dev.userid 
  and qa_contact is null

But in the second approach it excludes the result where qa_contact is null. Can anyone suggest a efficient way of doing this because I am dealing with records in order of millions and would like to add more filters on the resultset.

Answer 1

This is what LEFT JOINs are for:

SELECT bug_id, `desc`, dev.assigned_to, qa.qa_contact 
FROM bugs
INNER JOIN profiles as dev ON bugs.assigned_to = dev.userid
LEFT OUTER JOIN profiles as qa ON bugs.qa_contact = qa.userid

Answer 2

I think you want to use a LEFT JOIN:

select bug_id, desc, dev.assigned_to, qa.qa_contact 
from bugs b
left join profiles dev
  b.assigned_to dev.userid
left join profiles qa
  on b.qa_contact = qa.userid

If you need help learning JOIN syntax, then here is a great visual explanation of joins

A LEFT JOIN will return data from the bugs table even if the id does not exist in the profiles table.