There’s a MySQL pattern that I use fairly frequently that I want to share, both to help anyone else who might find it useful and also to find out if there’s a beter way.
Here’s an example: you have one table for users, another for posts, another for pages:
And we want to know how many posts and pages each user has:
Attempt 1: COUNT with JOIN
For the moment, lets focus on just getting the post count by user.
We might be tempted to try JOINing the two tables and using COUNT:
The problem is that when we JOIN the two tables, the results will only include users who have posts. In this case, there’s no result for Jen (user id 3) because she doesn’t have any records in the posts table.
Attempt 2: COUNT with LEFT JOIN
Ok, so if JOIN doesn’t work, lets try LEFT JOIN:
Now Jen has a count, but it’s reflecting 1 post, not 0 like we expect.
We can see why it’s broken by looking at the LEFT JOIN results without aggregating them:
Jen doesn’t have any posts, but because we’re LEFT JOINing, her users record is still included. When we then aggregate the results with GROUP BY and COUNT, MySQL sees that the results have one record so returns a count of 1.
A better way to do this (as suggested by Tom Davies) is instead of counting all records, only count post ids:
Attempt 3: SUM/IF, and LEFT JOIN
Another way we can achieve what we want (for a single table join) is to use SUM/IF:
Here we’re saying “When the post id is null, give it a 0, otherwise a 1, then sum the results” which gives us the correct count. This is actually I trick I learned from the real Jen while at Automattic :).
The problem arises when we need to join multiple tables:
Now instead of Simon (user id 2) having 2 posts, he has 4 – what happened? Again, we can look at the un-aggregated results:
The problem is that each post gets joined with each page result. Simon has 2 posts, each of which gets joined with the 2 pages, so when we use COUNT it sees 4 results and returns that amount.
The solution: Subqueries and COALESCE
Here’s how I would solve it:
To understand how it works, lets focus on counting posts. The first subquery counts how many posts each user has if they have any posts:
We can then LEFT JOIN users on this derived table:
For the users with posts, the result has the post count. For the users without posts, the result is NULL. This is where COALESCE comes into play. COALESCE takes any number of arguments and returns the first non-NULL result:
So we’re saying “If the user has a posts count, use that, otherwise use 0”.
We can then LEFT JOIN again on the pages table and do the same thing with the pages count to get the posts and pages count per user. Because each subquery only returns a max of one result per user, we don’t run into the issue we did earlier where posts get joined with pages to return the incorrect count.
Somewhat complicated, but the only way I know how to do it. If you know of a better way, please drop a comment below. Thanks!
This simpler method also works:
By counting distinct post ids and page ids, we avoid counting NULLs and also avoid counting duplicates due to joining on posts and pages.