Question

I have two tables..

1st Table: post

|post_id  |    post_data |
..........................
|   1     |    any data  |
|   2     |    any data  |
|   3     |    any data  |

2nd Table: post_likes

|like_id  |    post_id   | by_user |
....................................
|   1     |      1       |   3     |
|   2     |      3       |   3     |

when ever a user like any post data is stored in posts_likes table..
i want to show that posts (from both tables once) that are not liked by user 3..

i am using this query

SELECT *
FROM post, post_likes
WHERE post.post_id != post_likes.post_id
AND by_user=3

it showing me these results..

post_id     post_data   like_id     post_id     by_user
  1         my data       2            3          3
  2         my data       1            1          3
  2         my data       2            3          3
  3         my data       1            1          3

But it should show the result of post_id=2 only (because post_id 1 and 3 are liked by user)

what will be the correct query by which i can get those posts that are not liked by user 3

Answer 1

You can use a NOT IN ()subquery in your WHERE clause:

SELECT
  posts.post_id,
  posts.post_data,
  post_likes.like_id,
  post_likes.by_user
FROM posts LEFT JOIN post_likes ON posts.post_id = post_likes.post_id
WHERE posts.post_id NOT IN (
  SELECT DISTINCT post_id FROM post_likes WHERE by_user = 3
)