-- without using SET variable
SELECT t.id,
         t.count,
         (@running_total := @running_total + t.count) AS cumulative_sum
    FROM TABLE t
    JOIN (SELECT @running_total := 0) r
ORDER BY t.id

-- with SET variable
SET @running_total := 0;
SELECT
   q1.d,
   q1.c,
   (@running_total := @running_total + q1.c) AS rt
FROM
   (SELECT
       DAYOFYEAR(`date`) AS d,
       COUNT(*) AS c
    FROM  `orders`
    WHERE  `hasPaid` > 0
    GROUP  BY d
    ORDER  BY d
) AS q1