'num_of_cmt' is 4 instead of 2. Why? - mysql

Question

I have 4 tables:
users (id, name, email);
id | name | email
1 | ABC | abc#gmail.com
2 | XYZ | xyz#gmail.com
3 | AAA | aaa#yahoo.com
papers(id, title, content, created_by)
id | title | content | created_by
1 | This is title 1 | This is content 1 | 1
2 | This is title 2 | This is content 2 | 1
3 | This is title 3 | This is content 3 | 3
4 | This is title 4 | This is content 4 | 1
5 | This is title 5 | This is content 5 | 3
6 | This is title 6 | This is content 6 | 2
rating(id, paperId, star)
id | paperId | star
1 | 1 | 2
2 | 2 | 4
3 | 3 | 4
4 | 2 | 2
5 | 1 | 3
comments(id, paperId, msg)
id | paperId | msg
1 | 1 | abcd
2 | 2 | xxxx
3 | 2 | yyyy
4 | 3 | zzzz
5 | 1 | tttt
6 | 4 | kkkk
I want to get fields: papers.id, papers.title, papers.content, users.name,
AVG(rating.star), COUNT(comments.msg)
And I execute a query like:
SELECT papers.id, papers.title, papers.content, users.name,
AVG(rating.star) AS avg_star , COUNT(comments.msg) AS num_of_cmt
FROM papers
JOIN users ON users.id = papers.created_by
LEFT JOIN rating ON rating.paperId = papers.id
LEFT JOIN comments ON comments.paperId = papers.id
WHERE papers.id = 1
Then result is false at "num_of_cmt" field:
id title content name avg_star num_of_cmt
1 This is title 1 This is content 1 ABC 2.5000 4
Above, 'num_of_cmt' is 4 instead of 2. Why?

Answer 1

Both ratings and comments have multiple rows for paperid = 1. So, joining the tables yields four results, with the following ids:
ratings comments
1 1
1 5
5 1
5 5
Hence, the count is 4. You can fix the count by doing count(distinct comments.id). However, the average is going to be off.
One way to fix this problem is by aggregating ratings and comments in subqueries.