MySQL SUM() function dengan GROUP BY

MySQL SUM() function retrieves the sum value of an expression which has undergone a grouping operation by GROUP BY clause.

SELECT cate_id,SUM(total_cost)
FROM purchase
GROUP BY cate_id;

Source: http://www.w3resource.com/mysql/aggregate-functions-and-grouping/aggregate-functions-and-grouping-sum-with-group-by.php