树叶云OceanBase教程：OceanBase STDDEV

​STDDEV​ 函数用于计算总体标准差。​STDDEV​ 函数将数值型数据作为参数，返回数值型数据。它与函数 ​STDDEV_SAMP​ 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。

OceanBase 中标准差的值是函数 ​VARIANCE​ 计算出的方差的算术平方根。

语法

STDDEV([ DISTINCT | UNIQUE | 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

注意 
如果您指定了 ​
DISTINCT​ 或 ​
UNIQUE​ 关键字，则 ​
analytic_clause​ 中不允许出现 ​
order_by_clause​ 和 ​
windowing_clause​。

返回类型

返回 ​NUMBER​ 类型数据。

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 last_name, salary, STDDEV(salary) OVER (ORDER BY hiredate) "StdDev"
FROM employees WHERE manager_id = 100 ORDER BY last_name, salary, "StdDev";

查询结果如下：

+-----------+--------+-------------------------------------------+
| LAST_NAME | SALARY | StdDev                                    |
+-----------+--------+-------------------------------------------+
| De Haan   |  11000 | 4702.127178203498995615489088200868644482 |
| Errazuriz |   1400 |  212.132034355964257320253308631454711785 |
| Hartstein |  14000 | 6340.346993658943269176828928801701088079 |
| Partners  |  14000 | 6064.899009876421676804205219406952308814 |
| Raphaely  |   1700 |                                         0 |
| Raphaely  |   1700 |  173.205080756887729352744634150587236694 |
| Russell   |  13000 | 6026.474330580265330900400184969999384459 |
| Weiss     |  13500 | 6244.311697171159907069428668980211861012 |
+-----------+--------+-------------------------------------------+

聚合函数示例

调用函数并执行以下语句：

SELECT STDDEV(salary) FROM employees WHERE manager_id = 100 ;

查询结果如下：

+-------------------------------------------+
| STDDEV(SALARY)                            |
+-------------------------------------------+
| 6026.474330580265330900400184969999384459 |
+-------------------------------------------+