连接多个表并在MySQL中使用Count

Question

So I've got myself in a massive confused mess.

Basically, I have a little social network app and I have 4 different tables I need to combine into one view.

• Table 1 - User posts
• Table 2 - Users
• Table 3 - Likes
• Table 4 - Comments

I then need to return a list of posts, with the user details and then add columns for the number of likes and number of comments for each post respectively.

If a post doesn't have any likes or comments, then ideally we should show a Zero.

The query below joins everything up, but returns multiple rows of everything, as it is returning 1 row for each comment or like as well.

Anyone able to help me combine these together?

SELECT *
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
ORDER BY p.post_date DESC

Any help would be greatly appreciated!

Table columns are as follows;

app_likes

• like_id
• post_id
• user_id
• liked_date

app_comments

• comment_id
• comment_user_id
• post_id
• comment_body
• comment_date

app_posts

• post_id
• user_id
• post_content
• post_date
• post_public

app_user

• user_id
• user_first
• user_last
• user_avatar
• user_banned

An example of what is returned currently is as follows (chopped down for easiness)

You'll see the post_id is repeated multiple times.

What I want it to return is the post_id just once, and with the count of 'likes' and 'comments' in new columns (I don't know how to do this).

Simon

Answer 1

You might be missing GROUP BY...

SELECT p.*,u.*,count(distinct c.comment_id),count(distinct l.like_id)
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY p.post_id
ORDER BY p.post_date DESC

Note that MySQL lets you be sloppy with GROUP BY like this, but a lot of other databases would require you to break out the " p.* " into explicit MAX(p.post_id),MAX(p.post_content), etc.

Answer 2

SELECT p.post_id, COUNT(c.post_id) as num_comments, COUNT(l.like_id) as num_likes
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY  p.post_id
ORDER BY p.post_date DESC

Answer 3

Try to add Group by at the end of your query, like this

SELECT *
FROM app_posts AS p
LEFT JOIN app_comments AS c ON c.post_id = p.post_id
LEFT JOIN app_user AS u ON u.user_id = p.user_id
LEFT JOIN app_likes AS l ON l.post_id = p.post_id
WHERE u.user_banned = 0
AND p.post_public = 1
GROUP BY p.post_id
ORDER BY p.post_date DESC