Trouble with Joins and Counts in MySQL - mysql

Question

I am confused on this MySQL select query, I get the correct information back except the COUNT(messages) and COUNT(project_ideas) are coming back twice as many.
SELECT
create_project.title,
image1,
create_project.description,
create_project.date,
create_project.active,
create_project.completed,
create_project.project_id,
categories.name,
messages.receiver_read,
project_ideas.project_id,
COUNT(messages.ideas_id) AS num_of_messages,
COUNT(project_ideas.ideas_id) AS num_of_ideas
FROM
create_project
LEFT JOIN project_ideas ON create_project.project_id = project_ideas.project_id
LEFT JOIN messages ON messages.project_id = create_project.project_id
JOIN categories ON create_project.category = categories.category_id
WHERE
create_project.user_id = {$_SESSION['user']['user_id']}
AND create_project.active = 1
AND create_project.completed = 1
GROUP BY project_ideas.project_id
ORDER BY create_project.date ASC
Any help would be appreciated thanks.

Answer 1

If there is more than one row in your create_project table that matches to a single row in your messages table, then the row in messages will show up once for each matching row in create_project. Additionally, since you have many joins, you have many places for duplicate rows to show up. If a project belongs to more than one category, for example, your join against categories will result in every row from the other tables being duplicated for each category that a project belongs to. I'd wager this is actually the source of your error. And what makes it so insidious is that the GROUP BY hides the duplication everywhere except in functions that do counting and summing.
#Wrikken's comment is correct and useful. If you remove the GROUP BY, you'll see every row included in the count. There you should see that rows from the messages table are repeated. As #Wrikken also said, you can mitigate this by using COUNT(DISTINCT ...). I would try, however, to make sure your joins are correct or that your table data is correct, before papering over the problem with a COUNT(DISTINCT ...). That is to say, make sure that COUNT(DISTINCT ...) really makes logical sense in terms of the data you are looking for.
Unrelated to your action question, I had to point out something that I see (and have done myself before I knew better). Although MySQL lets you include columns in your select list that are not in a GROUP BY or an aggregate function (e.g., COUNT()), it's bad practice to do so. The results are technically undefined (see: http://dev.mysql.com/doc/refman/5.0/en/group-by-extensions.html). I think MySQL is wrong for doing this, but it's not my call. Other database systems would flag this as an error.

Answer 2

Try this:
COUNT(messages.ideas_id) OVER(PARTITION BY messages.project_id) AS num_of_messages,
COUNT(project_ideas.ideas_id) OVER(PARTITION BY project_ideas.project_id) AS num_of_ideas