Question

I am working on a project right now and am rather stumped with a specific sql query I (need) to execute. Let me start off by showing the DB structure I need to pull from.

    --posts_table--
    ID
    post_title
    post_text
    bus_id

This next table is what is screwing with me. The only way data related to the logged in user is in here is if they have “liked” a specific post — otherwise there is no data related to that user in this table. Now there could be plenty of data related to a particular post, just generated from other users.

    --likes_table--
    ID
    user_id
    post_id
    like

What I need this to do is grab all the posts from the post_table above where a specific business id is specified. From there, I need it to grab the “like” column in the likes_table if there is data in there related to the logged in user. If there is no data there, just leave that field null in the query. Below is a query I wrote that works until there is other “like” data in the like_table from other users.

    SELECT posts.id, posts.post_text, posts.post_title, likes.post_id, likes.like
        FROM posts LEFT JOIN likes ON posts.id = likes.post_id WHERE
        posts.bus_id = 1 AND likes.user_id IS NULL OR likes.user_id = 1;

This works up until data has been entered in the table about a specific post being liked by a different user before that user has done anything with that post, whether they like or dislike it. I am not sure if this specific type of query is even possible, any help would be much appreciated.

Edit:

After looking at it again — I got it, finally. I just needed to add one more AND. Below is the proper query I was looking for.

    SELECT posts.id, posts.post_text, posts.post_title, likes.post_id, likes.like
    FROM posts LEFT JOIN likes ON posts.id = likes.post_id AND posts.user_id = 1 WHERE
    posts.bus_id = 1 AND likes.user_id IS NULL OR likes.user_id = 1;

Answer 1

Ahh, I think I get you — is it that if a particular post hasn’t been commented on by user_id number 1 at all, the row for that doesn’t show up at all?

In that case, put your l.user_id=1 into the JOIN condition instead of the WHERE condition — this will put a NULL in if user_id 1 hasn’t liked or disliked a particular post.

SELECT p.id, p.post_text, p.post_title, l.post_id, l.likes
  FROM posts p
  LEFT JOIN likes l ON p.id = l.post_id AND l.user_id=1
  WHERE p.bus_id = 1 

The l.user_id IS NULL OR l.user_id=1 has been incorporated into the LEFT JOIN — it doesn’t make rows for the other user_ids.