MySQL query error: file - , line -

SELECT a.categoryID,, count(*) as num FROM sewc_shop_category a, sewc_shop_products b, sewc_shop_products c WHERE a.categoryID=b.category2ID AND b.productID=c.productID # AND b.enable = '1' GROUP BY c.category2ID ORDER BY a.categoryID

Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'sewc_ru.a.categoryID' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by