STDDEV_SAMP 函数计算样本标准差。STDDEV_SAMP 函数将数值型数据作为参数，返回数值型数据。它与函数 [STDDEV](https://www.oceanbase.com/docs/apsaradb-for-oceanbase/docs/V2.2.30/lbfuea) 的不同之处在于，STDDEV 只有一行输入数据时返回 0，而 STDDEV_SAMP 返回 NULL。

说明

样本标准差是样本方差的算术平方根。

语法

STDDEV_SAMP([ALL] expr) [ OVER (analytic_clause) ]

作为分析函数使用时，您需要使用窗口函数的完整语法，它对一组行的集合进行计算并返回多个值 。作为聚合函数使用时，该函数对一组行的集合进行聚合计算，结果只能返回一个值，此时不需要加 OVER 关键字。

参数

返回类型

返回类型与参数 expr 的数据类型相同。

示例

分析函数示例

以下语句创建了表 employees，并向里面插入数据：

CREATE TABLE employees (manager_id INT,last_name varchar(50),hiredate varchar(50),SALARY INT);
INSERT INTO employees VALUES(100, 'Raphaely', '2017-07-01', 1700);
INSERT INTO employees VALUES(100, 'De Haan', '2018-05-01',11000);      
INSERT INTO employees VALUES(100, 'Errazuriz', '2017-07-21', 1400);
INSERT INTO employees VALUES(100, 'Hartstein', '2019-05-01',14000);     
INSERT INTO employees VALUES(100, 'Raphaely', '2017-07-22', 1700);
INSERT INTO employees VALUES(100, 'Weiss',  '2019-07-11',13500);     
INSERT INTO employees VALUES(100, 'Russell', '2019-10-05', 13000);
INSERT INTO employees VALUES(100, 'Partners',  '2018-12-01',14000);     
INSERT INTO employees VALUES(200, 'Ross',  '2019-06-11',13500);     
INSERT INTO employees VALUES(200, 'Bell', '2019-05-25', 13000);
INSERT INTO employees VALUES(200, 'Part',  '2018-08-11',14000);
COMMIT;

调用函数并执行以下语句：

SELECT manager_id, last_name, hiredate, salary,STDDEV_SAMP(salary) OVER (PARTITION BY manager_id
ORDER BY hiredate ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cum_sdev 
FROM employees ORDER BY manager_id, last_name, hiredate, salary, cum_sdev;

返回结果如下：

+------------+-----------+------------+--------+-------------------------------------------+
| MANAGER_ID | LAST_NAME | HIREDATE   | SALARY | CUM_SDEV                                  |
+------------+-----------+------------+--------+-------------------------------------------+
|        100 | De Haan   | 2018-05-01 |  11000 | 4702.127178203498995615489088200868644482 |
|        100 | Errazuriz | 2017-07-21 |   1400 |  212.132034355964257320253308631454711785 |
|        100 | Hartstein | 2019-05-01 |  14000 | 6340.346993658943269176828928801701088079 |
|        100 | Partners  | 2018-12-01 |  14000 | 6064.899009876421676804205219406952308814 |
|        100 | Raphaely  | 2017-07-01 |   1700 |                                      NULL |
|        100 | Raphaely  | 2017-07-22 |   1700 |  173.205080756887729352744634150587236694 |
|        100 | Russell   | 2019-10-05 |  13000 | 6026.474330580265330900400184969999384459 |
|        100 | Weiss     | 2019-07-11 |  13500 | 6244.311697171159907069428668980211861012 |
|        200 | Bell      | 2019-05-25 |  13000 |  707.106781186547524400844362104849039285 |
|        200 | Part      | 2018-08-11 |  14000 |                                      NULL |
|        200 | Ross      | 2019-06-11 |  13500 |                                       500 |
+------------+-----------+------------+--------+-------------------------------------------+

聚合函数示例

调用函数并执行以下语句：

SELECT STDDEV_SAMP(salary) FROM employees ;

查询结果如下：

+-------------------------------------------+
| STDDEV_SAMP(SALARY)                       |
+-------------------------------------------+
| 5506.194858355615640082358245403620332764 |
+-------------------------------------------+