如何正确编写子查询以获取每个帖子的评论数？

Question

I'm trying to return a sqlite3 query that will allow me to loop through social media posts and display the post and user details and the number of comments on each post.

I can't figure out how to write a sql query that gets the number of comments for the post.

Here is my database scheme in ERD:

And here is my query:

SELECT u.name, u.icon, p.date, p.title, p.message,
       (SELECT comment.postId, COUNT(comment.Id) 
        FROM comment
        WHERE comment.postID = p.Id) as commentno
FROM Post p, User u
WHERE p.userID = u.Id

What I see when I try to run this:

Error: sub-select returns 2 columns - expected 1

What I want to see:

Michael Scott, Michael.png, 2019-08-27, "Blog title", "Blog message", 7

Answer 1

See my comment above. I think you intended this:

    SELECT u.name, u.icon, p.date, p.title, p.message,
       c.postId, COUNT(c.Id) as commentno             
        FROM Post p LEFT JOIN  User u ON p.userID = u.Id
        LEFT JOIN Comment c on p.ID = c.PostID
        GROUP BY  u.name, u.icon, p.date, p.title, p.message, c.postId

UPDATE As was pointed out, my original answer had the same error (because I just quickly moved the alias. Here I reformatted your query to use JOINs . I assumed that your Post table has a comment ID, but you can figure out the appropriate columns for your JOIN .