MySQL Join and excluding rows with count=0 - php

Question

I have the following straightforward query which is part of a garbage collection script. The script is supposed to delete unused shopping carts. A cart is unused when it was updated more than 24 hours ago and is not attached to an order or a user.
$query = "SELECT comm_cart.id AS `cart_id`, (" . time() . " - comm_cart.update_date) AS `diff`, COUNT(comm_orders.cart_id) AS `c1`, COUNT(comm_users.cart_id) AS `c2` " .
"FROM `comm_cart` " .
"LEFT JOIN `comm_orders` ON comm_cart.id=comm_orders.cart_id " .
"LEFT JOIN `comm_users` ON comm_cart.id=comm_users.cart_id " .
"GROUP BY comm_cart.id ";
"HAVING `diff`>86400 AND `c1`=0 AND `c2`=0";
The query finds too many carts: it also tags the ones that have a c1>0 or c2>0 and I can't figure out why. Any clues?

Answer 1

I suspect that you are joining along two different dimensions. The easy fix is to use distinct:
SELECT comm_cart.id AS `cart_id`, (" . time() . " - comm_cart.update_date) AS `diff`,
COUNT(DISTINCT comm_orders.cart_id) AS `c1`, COUNT(DISTINCT comm_users.cart_id) AS `c2` " .
The better solution would be to use not exists for these two conditions:
FROM comm_carts cc
WHERE not exists (select 1 from comm_orders co where cc.id = co.cart_id )
not exists (select 1 from comm_users cu where cc.id = cu.cart_id )

Answer 2

You don't even need the group by, an isnull condition in the where would work wonders, of course I suggest using Gordon's suggestion of the not exists, but if you want the minimum change this would be is.
SELECT
comm_cart.id AS `cart_id`,
(UNIX_TIMESTAMP() - comm_cart.update_date) AS `diff`
FROM `comm_cart`
LEFT JOIN `comm_orders`
ON comm_cart.id=comm_orders.cart_id
LEFT JOIN `comm_users`
ON comm_cart.id=comm_users.cart_id
WHERE
comm_orders.cart_id IS NULL
AND
comm_users.cart_id IS NULL
Oh, and I've used UNIX_TIMESTAMP() instead of the PHP time function, same effect, but this avoids mixing PHP and SQL.

Answer 3

if you only want to get data with c1=0 and c2=0 then you need to write a where condition instead of having before group by,
$query = "SELECT comm_cart.id AS `cart_id`, (" . time() . " - comm_cart.update_date) AS `diff`, COUNT(comm_orders.cart_id) AS `c1`, COUNT(comm_users.cart_id) AS `c2` " .
"FROM `comm_cart` " .
"LEFT JOIN `comm_orders` ON comm_cart.id=comm_orders.cart_id " .
"LEFT JOIN `comm_users` ON comm_cart.id=comm_users.cart_id " .
" where c1=0 and c2 =0 and diff >86400 GROUP BY comm_cart.id;