使用SQL JOIN和COUNT

Question

Let there be two tables, one holding user information and one holding user records of some sort, say receipts. There is a one-to-many relationship between the users and receipts.

What would be the best SQL method of retrieving users, sorted by the greatest number of receipts?

The best way I can think of is using a join and count(?) to return an array of users and their number of associated receipts.

Is there a way to make use of the count function in this instance?

select * from `users` inner join `receipts` on `users`.`id` = `receipts`.`uId`

Answer 1

If OP wishes to include additional information (additional aggregations, etc...) utilizing data from users table:

SELECT `users`.`id`,
       count(`receipts`.`uId`)
FROM `users`
INNER JOIN `receipts` ON `users`.`id` = `receipts`.`uId`
GROUP BY `users`.`id`
ORDER BY count(`receipts`.`uId`) DESC

Otherwise, only the receipts table is required...

SELECT `users`.`id`,
       count(`receipts`.`uId`)
FROM `receipts`
GROUP BY `receipts`.`uId`
ORDER BY count(`receipts`.`uId`) DESC

Answer 2

try this

SELECT a.`id`, count(b.`recipts`) as total_receipts
FROM `users` a
INNER JOIN `receipts` b
ON a.`id` = b.`uId`
GROUP BY a.`id` 
ORDER BY count(b.`receipts`) desc

Answer 3

SELECT users.*, (SELECT COUNT(*) FROM tblreceipts WHERE tblreciepts.uId=users.id) as counter FROM users ORDER BY counter DESC

Something like this may work (not sure on the speed though if its big tables)

Answer 4

If you want to include all users, even those with no receipts, then a good way is a left outer join :

SELECT u.*, count(r.uid) as NumReceipts
FROM `users` u left outer join
     `receipts` r
    ON u.id = r.`uId
GROUP BY `u.id
ORDER BY NumReceipts DESC;

If you only want the id for users that have receipts, then the join is not even necessary:

SELECT r.uid, count(*) as NumReceipts
FROM receipts r
GROUP BY r.uid
ORDER BY NumReceipts

Answer 5

Two answers provided by Dave and meewoK will accomplish what you need. I'm providing an alternative, which should provide better performance and allow you to show more user information because in the case with Dave's answer you can only SELECT columns that are used by an aggregate function or in the group clause.

SELECT users.id, users.name, r.numReceipts  
FROM users u
INNER JOIN (
   SELECT uId, count(receipts) as numReceipts
   FROM receipts
   GROUP BY receipts.id
) as r ON r.uId = u.id
ORDER BY r.numReceipts DESC

This creates an inline view. Only return the count of receipts of each user and then join this inline view on the user's ID.

Some one correct me if I'm wrong, but I've been told that the planner isn't as efficient when you do a scalar subquery in the SELECT clause. It's better to join on a temporary table this way. There are multiple ways to write this query and it all depends on how you want to use the information!!! Cheers!