Question

(SELECT posts.id FROM posts 
INNER JOIN discussions ON discussions.post_id = posts.id 
INNER JOIN companies ON discussions.company_id = companies.id 
INNER JOIN subscriptions ON subscriptions.subscribable_id = companies.id AND subscriptions.subscribable_type = 'Company' 
INNER JOIN users ON subscriptions.user_id = users.id WHERE users.id = 6)

UNION 

(SELECT posts.id FROM posts 
INNER JOIN users users_2 ON posts.analyst_id = users_2.id 
INNER JOIN subscriptions ON subscriptions.subscribable_id = users_2.id AND subscriptions.subscribable_type = 'User'
INNER JOIN users ON subscriptions.user_id = users.id WHERE users.id = 6)

It should be obvious that the last join is the same in both queries.. Just not sure how to “or” together joins.

Answer 1

Some joins seem redundant:

• discussions could be joined to subscriptions directly on the company_id column;

• posts could be joined to subscriptions directly on the analyst_id column;

• the last join to users in either SELECT is unnecessary as no data is retrieved from that table and the filter (users.id = 6) could be re-applied to subscriptions.user_id.

So, I would probably rewrite the query like this:

SELECT p.id
FROM posts p
  INNER JOIN discussions d ON d.post_id = p.id
  INNER JOIN subscription s
    ON s.subscribable_type = 'Company' AND s.subscribable_id = d.company_id
    OR s.subscribable_type = 'User'    AND s.subscribable_id = p.analyst_id
WHERE s.user_id = 6