树叶云OceanBase教程：OceanBase AVG

AVG 函数返回数值列的平均值。

语法

AVG([ 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, '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;

执行以下语句计算各列的平均值：

SELECT manager_id, last_name, hiredate, salary, AVG(salary) OVER (PARTITION BY manager_id 
ORDER BY hiredate ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS c_mavg
FROM employees ORDER BY manager_id, hiredate, salary;

返回结果如下：

+------------+-----------+------------+--------+------------------------------------------+
| MANAGER_ID | LAST_NAME | HIREDATE   | SALARY | C_MAVG                                   |
+------------+-----------+------------+--------+------------------------------------------+
|        100 | Errazuriz | 2017-07-21 |   1400 |                                     1550 |
|        100 | Raphaely  | 2017-07-22 |   1700 |                                     4700 |
|        100 | De Haan   | 2018-05-01 |  11000 |                                     8900 |
|        100 | Partners  | 2018-12-01 |  14000 |                                    13000 |
|        100 | Hartstein | 2019-05-01 |  14000 |                                13833.333 |
|        100 | Weiss     | 2019-07-11 |  13500 |                                    13500 |
|        100 | Russell   | 2019-10-05 |  13000 |                                    13250 |
|        200 | Part      | 2018-08-11 |  14000 |                                    13500 |
|        200 | Bell      | 2019-05-25 |  13000 |                                    13500 |
|        200 | Ross      | 2019-06-11 |  13500 |                                    13250 |
+------------+-----------+------------+--------+------------------------------------------+

聚合函数示例

执行以下语句计算 salary 的平均值：

SELECT AVG(salary) FROM employees;

查询结果如下：

+-------------+
| AVG(SALARY) |
+-------------+
|       10910 |
+-------------+