Question

I’m having a problem which I easily solve with php and some queries, but I wanted to make one big query (at least if it’s faster which I believe it should be).

So I have 3 tables and let simplify them some:

topic which has 3 columns

• id
• user_id
• visibility

access which has 2 columns

• topic_id
• user_id

friendship which has 2 columns

• user_id
• friend_id

What I want to do is if a user (we call him watch_id) is trying to watch this topic I want to return the topic if he is allowed to watch or if he is not, return nothing.
He is allowed to watch if any of these is true:

• watch_id == user_id
• visibility == 3
• visiblity == 2 && the friendship table returns a row when friendship.user_id = topic.user_id && friendship.friend_id == watch_id
• visibility == 1 && the access table returns a row when access.topic_id = topic.id && access.user_id == watch_id

As you can see it’s not very hard to do with php and a bunch of queries but in sql only I can’t figure it out. I’ve been trying joins, case and stuff but the logic never adds up. 🙁

So can you guys help me? Or am I stuck with php and lots of queries?

Edit: Hmm, looks like I wasn’t making myself perfectly clear, my bad! He is allowed to watch if any of those requirements is true. Since this is the case I go with left join answer which was easiest to extend. Thanks all!

Answer 1

You should be able to make left joins against the friendship and access tables, so that the joins themselves doesn’t limit the result:

select something
from topic
left join friendship on friendship.user_id = topic.user_id && friendship.friend_id == watch_id
left join access on access.topic_id = topic.id && access.user_id == watch_id
where
  watch_id == topic.user_id or
  topic.visibility == 3 or
  (topic.visiblity == 2 and friendship.user_id is not null) or
  (visibility == 1 && access.topic_id is not null)