Optimizing MySQL query removing subquery - mysql

Question

Having these tables:
customers
---------------------
`id` smallint(5) unsigned NOT NULL auto_increment,
`name` varchar(100) collate utf8_unicode_ci default NOT NULL,
....
customers_subaccounts
-------------------------
`companies_id` mediumint(8) unsigned NOT NULL,
`customers_id` mediumint(8) unsigned NOT NULL,
`subaccount` int(10) unsigned NOT NULL
I need to get all the customers whom have been assigned more than one subaccount for the same company.
This is what I've got:
SELECT * FROM customers
WHERE id IN
(SELECT customers_id
FROM customers_subaccounts
GROUP BY customers_id, companies_id
HAVING COUNT(subaccount) > 1)
This query is too slow though. It's even slower if I add the DISTINCT modifier to customers_id in the SELECT of the subquery, which in the end retrieves the same customers list for the whole query. Maybe there's a better way without subquerying, anything faster will help, and I'm not sure whether it will retrieve an accurate correct list.
Any help?

Answer 1

You can replace the subquery with an INNER JOIN:
SELECT t1.id
FROM customers t1
INNER JOIN
(
SELECT DISTINCT customers_id
FROM customers_subaccounts
GROUP BY customers_id, companies_id
HAVING COUNT(*) > 1
) t2
ON t1.id = t2.customers_id

Answer 2

You can also try using EXISTS() which may be faster then a join :
SELECT * FROM customers t
WHERE EXISTS(SELECT 1 FROM customers_subaccounts s
WHERE s.customers_id = t.id
GROUP BY s.customers_id, s.companies_id
HAVING COUNT(subaccount) > 1)
You should also considering adding the following indexes(if not exists yet) :
customers_subaccounts (customers_id,companies_id,subaccount)
customers (id)

Answer 3

Assuming that you want different subaccounts for the company (or that they are guaranteed to be different anyway), then the following could be faster under some circumstances:
select c.*
from (select distinct cs.customers_id
from customers_subaccounts cs join
customers_subaccounts cs2
on cs.customers_id = cs2.customers_id and
cs.companies_id = cs2.companies_id and
cs.subaccount < cs2.subaccount
) cc join
customers c
on c.customers_id = cc.customers_id;
In particular, this can take advantage of an index on customers_subaccounts(customers_id, companies_id, subaccount).
Note: This assumes that the subaccounts are different for the rows you want. What is really needed is a way of defining unique rows in the customers_subaccounts table.

Answer 4

There is a way to speed up the query by using cache the sub-query result. A simple change in your query aware mysql that can cache the sub-query result:
SELECT * FROM customers
WHERE id IN
(select * from
(SELECT distinct customers_id
FROM customers_subaccounts
GROUP BY customers_id, companies_id
HAVING COUNT(subaccount) > 1) t1);
I used it many years ago and it helped me very much.

Answer 5

Try following;)
SELECT DISTINCT t1.*
FROM customers t1
INNER JOIN customers_subaccounts t2 ON t1.id = t2.customers_id
GROUP BY t1.id, t1.name, t2.companies_id
HAVING COUNT(t2.subaccount) > 1
Also you may add index on customers_id.