Question

With the following tables

posts

• id
• post_id
• user_id

comments

• id
• post_id
• comment_id
• user_id
• deleted

replies

• id
• post_id
• reply_id
• user_id
• deleted

I am trying to get every comment and reply from each post.post_id and post.user_id=’x’ and the comment or reply is not deleted(0)

this is what i have tried

SELECT *
FROM posts p
LEFT JOIN comments c ON p.id=c.post_id
LEFT JOIN replies r ON p.id=r.post_id
WHERE p.user_id=$user_id
&& c.deleted='0' && r.deleted='0'

which does not work…

Answer 1

A post may have comments or not. Use LEFT JOIN instead of INNER JOIN.

A post may have replies or not. Use LEFT JOIN instead of INNER JOIN in that join too.

When LEFT JOIN is used, a condition like WHERE comments.deleted = 0 that includes a field (from the right table (comments) in the LEFT JOIN), the LEFT JOIN is cancelled. So, we should put this condition in the ON clause and not in the WHERE.

SELECT *
FROM posts p
  LEFT JOIN comments c
    ON p.post_id = c.post_id
    AND c.deleted = 0
  LEFT JOIN replies r
    ON p.post_id = r.post_id
    AND r.deleted = 0  
WHERE p.user_id = $user_id

---

Thinking more clearly, the above will show what the question describes but in cases with say, 4 comments and 3 replies, 12 rows will be returned (3×4). Which is probably not wanted. The following 2nd try does not have such issue.

I don’t see a post.text or comment.text or reply.text in the tables but anyway, you’ll get the idea. You can remove the 3 text lines if not appropriate.

  ( SELECT p.post_id     AS post_id 
         , 0             AS type
         , p.post_id     AS id
         , p.text        AS text
    FROM posts p
    WHERE p.user_id = $user_id
  )
UNION ALL
  ( SELECT p.post_id     AS post_id 
         , 1             AS type
         , c.comment_id  AS id
         , c.text        AS text
    FROM posts p
      JOIN comments c
        ON p.post_id = c.post_id
    WHERE p.user_id = $user_id
      AND c.deleted = 0
  )
UNION ALL
  ( SELECT p.post_id     AS post_id 
         , 2             AS type
         , r.reply_id    AS id
         , r.text        AS text
    FROM posts p
      JOIN replies r
        ON p.post_id = r.post_id
    WHERE p.user_id = $user_id
      AND r.deleted = 0
  )
ORDER BY post_id
       , post_type
       , id

The 0,1,2 stand for post, comment, reply.