Could not execute query

[SELECT COUNT(UP.USERID) AS TOTAL, UP.USERID AS USERID, ROUND(AVG(IFNULL(URT.RATING, -1)), 0) AS RATING FROM PRICECOMPARISON_USER U, PRICECOMPARISON_CLICKTHROUGHS UR, PRICECOMPARISON_USER_PRODUCT UP LEFT OUTER JOIN PRICECOMPARISON_USER_RATING URT ON UP.USERID=URT.MERCHANTID AND URT.ISVALID='Y' WHERE U.USERID = UP.USERID AND U.ISVALID = 'Y' AND UR.USER_PRODUCT_ID = UP.ID AND UR.ISMERCHANT='N' GROUP BY UP.USERID ORDER BY TOTAL DESC LIMIT 0, 25] - Lost connection to MySQL server during query