mysql之 sum() over(partition by)

SELECT empno,
ename,
sal,
IF(@deptno = deptno, @sal := @sal, @sal := s_sal) AS s_sal,
@deptno := deptno
FROM
(
SELECT empno,
ename,
sal,
IF(@deptno = deptno, @sal := @sal + sal, @sal := sal) AS s_sal,
@deptno := deptno AS deptno
FROM (SELECT @sal := 0, @deptno := 0) a0, emp t
ORDER BY deptno, empno
)e
ORDER BY deptno, empno DESC;

+-------+--------+------+-------+-------------------+
| empno | ename  | sal  | s_sal | @deptno := deptno |
+-------+--------+------+-------+-------------------+
|  7934 | MILLER | 1300 |  8750 |                10 |
|  7839 | KING   | 5000 |  8750 |                10 |
|  7782 | CLARK  | 2450 |  8750 |                10 |
|  7902 | FORD   | 3000 | 10875 |                20 |
|  7876 | ADAMS  | 1100 | 10875 |                20 |
|  7788 | SCOTT  | 3000 | 10875 |                20 |
|  7566 | JONES  | 2975 | 10875 |                20 |
|  7369 | SMITH  |  800 | 10875 |                20 |
|  7900 | JAMES  |  950 |  9400 |                30 |
|  7844 | TURNER | 1500 |  9400 |                30 |
|  7698 | BLAKE  | 2850 |  9400 |                30 |
|  7654 | MARTIN | 1250 |  9400 |                30 |
|  7521 | WARD   | 1250 |  9400 |                30 |
|  7499 | ALLEN  | 1600 |  9400 |                30 |
+-------+--------+------+-------+-------------------+
14 rows in set (0.01 sec)