Question

Let’s say I have a posts table that has many comments (and a comment belongs to a user).

How would I show all posts where a particular user hasn’t commented on?

Pretend I have a user in my database with an ID of 124;

select * from posts p left join comments c on p.id = c.post_id 
where c.id is null or c.user_id <> 124

But that doesn’t seem right .. as what if the post has no comments?

Answer 1

You should be checking the user_id from table post and not on table comments because there are some user_id that doesn’t have any records yet on table comments.

SELECT  p.* 
FROM    posts p
        LEFT JOIN comments c
            ON p.id = c.post_id AND c.user_id = 124
WHERE   c.post_id IS NULL