How to get data from table by intermediaries - mysql

Question

I have three table:
// users
+----+------+------------+
| id | name | reputation |
+----+------+------------+
// posts
+----+-------+---------+
| id | title | user_id |
+----+-------+---------+
// votes
+----+---------+---------+
| id | user_id | post_id |
+----+---------+---------+
Note: user_id in votes is belong to who gives a vote. But user_id in posts table is belong to who wrote that post.
So I want to give +5 rep to the owner of post when his comment get a upvote.
Example: when userA gives a upvote to the post (wroted by userB), I want to run this:
update users set reputatuin=reputation+5 where id = {how get the id of userB}
Now I want to know, how should I obtain the userB (post owner who wrote it) id ?

Answer 1

In an UPDATE statement, you must join through posts to the votes table using MySQL's multi-table UPDATE syntax.
If you want to update by targeting the votes.id of the new vote, use that in the WHERE clause.
UPDATE
users
INNER JOIN posts ON users.id = posts.user_id
INNER JOIN votes ON votes.post_id = posts.id
SET
users.reputation = users.reputation + 5
WHERE votes.id = {vote id to update}
If your code is already aware of the posts.id of the voted post, then the votes table need not be joined and you can use users and posts.
UPDATE
users
INNER JOIN posts ON users.id = posts.user_id
SET
users.reputation = users.reputation + 5
WHERE posts.id = {post id of new vote}
This query can be done just as easily with a subquery in the WHERE clause.
UPDATE users
SET reputation = reputation + 5
WHERE
id = (SELECT user_id FROM posts WHERE post_id = {post id of new vote})