performance problem

[Babak Rafat]

Hi,
I have a fairly simple query and a base table named "customers" with around 900000 record.
when I run this query in best of it I have to wait 9 seconds to get the result and this is too much.

This is the query :

SELECT customers.item, COUNT(customers.item) AS totalqty
FROM
report_history
LEFT JOIN user ON user.id = customers.userId
LEFT JOIN usergroup ON user.usergroupId = usergroup.id
WHERE
user.id LIKE '%%'
AND
(user.userGroupId = '0' OR usergroup.level LIKE '%')
AND
buyDateTime BETWEEN '2011-01-01 00:00:00' AND '2012-05-02 23:59:59'

GROUP BY
customers.item
ORDER BY
totalqty DESC
limit 20;

Please help me to optimize this query.