SQL SELECT and COUNT of second table

Question

Having 2 tables:

users
==========
id
name

notes
=========
id
user_id
note

Users can have multiple notes.

I would like to do a SQL query that gets all users with all their fields but also the amount of notes. Further I would like the result to be ordered by the amount of notes (desc)

I wrote this, but I want to know if it's is the correct/fastest way to solve this:

SELECT u.*, count(n.id) as amountnotes FROM users AS u 
LEFT JOIN notes AS n ON u.id=n.user_id
GROUP BY u.id
ORDER BY amountnotes DESC

EXPLAIN gives me 2 results. "Using temporary; Using filesort" on users and "Using where; Using join buffer (Block Nested Loop)" on notes

Any help appreciated, thanks. If suggesting a different approach please explain why so I can understand.

Answer 1

If COUNT is all you want, you can do it with a subquery rather than a join, like this:

SELECT u.*
,      (SELECT COUNT(*) FROM notes WHERE notes.user_id=u.id) as amountnotes
FROM users u
ORDER BY amountnotes DESC

If you do it the JOIN way, your GROUP BY should list all columns of the user (although MySql is one of the RDBMS systems that does not require this, porting such SQL to databases of other vendors becomes problematic).

Answer 2

Your query will return the resultset you specified, assuming that id is UNIQUE (or PRIMARY) KEY in users table. Your query looks fine for MySQL.

Best practice is to list out the expressions you want returned, rather than using the *; but what you have is valid.

Note that this query will work in MySQL, but other databases will throw an error, because there are non-aggregates in the SELECT list that are not included in the GROUP BY.

(Your query isn't the only way to get the specified resultset, there are several alternatives).