多对多查询

Question

I have the following tables:

user (id, name, email) follow (id, follower_id, following_id) post (id, user_id, title, body)

Now follow means I (user_id = 23) follow another user (user_id = 12) and maybe a few more.

I want to make a query where it can display ALL posts made by ALL people I follow.

Is it possible with a single Query?

Answer 1

Absolutely, however make sure you have proper indexes otherwise it may take a while!

SELECT `user`.`id` AS `user_id`, `user`.`name`, `post`.`id` AS `post_id`, `post`.`title`, `post`.`body`
FROM `follow`
JOIN `user` ON `user`.`id`=`follow`.`following_id`
JOIN `post` ON `post`.`something...` # You're missing an "author" field in your posts table!
WHERE `follow`.`following_id` = 23

Answer 2

You don't mention it in the question, but you gotta have a column such as poster_id that identifies who made the post in your Post table. Having that, you can do:

SELECT p.* 
FROM posts p
INNER JOIN follow f ON p.poster_id = f.following_id
WHERE f.follower_id = 23;

This will give you all the posts from the users followed by user 23.