oracle SQL 语句向MYSQL迁移问题

有个oracle语句：
SELECT employee_id, salary, hire_date, STDDEV(salary)
OVER (ORDER BY hire_date) "Std Deviation of Salary"
FROM employees WHERE job_id = 'ST_CLERK';执行结果如下：
EMPLOYEE_ID     SALARY HIRE_DATE  Std Deviation of Salary
----------- ---------- ---------- -----------------------
        137       3600 14-7月 -95                       0
        141       3500 17-10月-95              70.7106781
        133       3300 14-6月 -96              152.752523
        142       3100 29-1月 -97              221.735578
        131       2500 16-2月 -97              435.889894
        125       3200 16-7月 -97              389.871774
        129       3300 20-8月 -97              357.903951
        138       3200 26-10月-97              331.393163
        130       2800 30-10月-97              339.116499
        139       2700 12-2月 -98              352.136337
        143       2600 15-3月 -98              369.028208EMPLOYEE_ID     SALARY HIRE_DATE  Std Deviation of Salary
----------- ---------- ---------- -----------------------
        140       2500 06-4月 -98              388.762606
        144       2500 09-7月 -98              399.679359
        134       2900 26-8月 -98              384.664832
        126       2700 28-9月 -98              377.586319
        127       2400 14-1月 -99               390.72582
        132       2100 10-4月 -99              427.974023
        135       2400 12-12月-99              430.116263
        136       2200 06-2月 -00              443.800703
        128       2200 08-3月 -00              453.379126我写了一个对应的mysql语句：
SELECT employee_id, salary, hire_date, STDDEV_pop(salary)
"Std Deviation of Salary"
FROM employees WHERE job_id = 'ST_CLERK' 
Group by employee_id, salary, hire_date
ORDER BY hire_date;
执行结果如下:
+-------------+--------+------------+-------------------------+
| employee_id | salary | hire_date  | Std Deviation of Salary |
+-------------+--------+------------+-------------------------+
| 137         | 3600   | 1995-07-14 | 0.0000                  |
| 141         | 3500   | 1995-10-17 | 0.0000                  |
| 133         | 3300   | 1996-06-14 | 0.0000                  |
| 142         | 3100   | 1997-01-29 | 0.0000                  |
| 131         | 2500   | 1997-02-16 | 0.0000                  |
| 125         | 3200   | 1997-07-16 | 0.0000                  |
| 129         | 3300   | 1997-08-20 | 0.0000                  |
| 138         | 3200   | 1997-10-26 | 0.0000                  |
| 130         | 2800   | 1997-10-30 | 0.0000                  |
| 139         | 2700   | 1998-02-12 | 0.0000                  |
| 143         | 2600   | 1998-03-15 | 0.0000                  |
| 140         | 2500   | 1998-04-06 | 0.0000                  |
| 144         | 2500   | 1998-07-09 | 0.0000                  |
| 134         | 2900   | 1998-08-26 | 0.0000                  |
| 126         | 2700   | 1998-09-28 | 0.0000                  |
| 127         | 2400   | 1999-01-14 | 0.0000                  |
| 132         | 2100   | 1999-04-10 | 0.0000                  |
| 135         | 2400   | 1999-12-12 | 0.0000                  |
| 136         | 2200   | 2000-02-06 | 0.0000                  |
| 128         | 2200   | 2000-03-08 | 0.0000                  |
+-------------+--------+------------+-------------------------+请问问题出在哪里？