join 3 sql tables and make nested query - mysql

Question

So I have 3 tables called comments, users and posts.
I want to get
the "score" from comments
the "user reputation" for users by doing the left join between comments and users with c.user_id = u.id
get the "tags" from posts table by doing a left join between comments and posts on c.post_id = p.id.
BUT there is a trick here the tags should be based on the type of the posts (p.post_type_id).
So if the id = 1 then that means we have a "question" as a post and simply retrieve tag
Else if the id = 2 then that means we have an answer and to get the tag we have to look at its parent_id from posts table.
I tried to use WHERE, CASE, nested IF, and nested SELECT but all threw syntax errors
Finally, I tried to do the following but I got an empty result
SELECT c.score,
COALESCE (u.reputation) reputation,
COALESCE (p.tags) tags
FROM comments c
LEFT JOIN users u
ON c.user_id = u.id
LEFT JOIN posts p
ON (c.post_id = p.id AND p.post_type_id = 1) OR (c.post_id = p.id AND p.post_type_id = 2 )
WHERE (p.id = p.parent_id)
So how can I have the tags based on the two types ?

Answer 1

Just quick guess:
http://sqlfiddle.com/#!9/a1cc3/1
SELECT c.score,
u.reputation reputation,
IF(p.post_type_id=1,p.tags,
IF(p.post_type_id=2,parents.tags,'UNKNOWN POST TYPE')
) tags
FROM comments c
LEFT JOIN users u
ON c.user_id = u.id
LEFT JOIN posts p
ON c.post_id = p.id
LEFT JOIN posts parents
ON parents.id = p.parent_id
UPDATE Here is Postgres variant:
http://sqlfiddle.com/#!15/a1cc3/2
SELECT c.score,
u.reputation,
CASE p.post_type_id
WHEN 1 THEN p.tags
WHEN 2 THEN parents.tags
ELSE 'UNKNOWN POST TYPE'
END tags
FROM comments c
LEFT JOIN users u
ON c.user_id = u.id
LEFT JOIN posts p
ON c.post_id = p.id
LEFT JOIN posts parents
ON parents.id = p.parent_id