APPROX_COUNT_DISTINCT 函数是聚合函数，它对某一列去重后的行数进行计算，结果只能返回一个值，且该值是近似值，该函数可以进一步用于计算被引用的列的选择性。

与函数 COUNT(DISTINCT x) 相比，APPROX_COUNT_DISTINCT 返回的是近似值，所以计算速度极快。在处理大量级的数据时 COUNT(DISTINCT x) 经常要花费很长的时间，使用 APPROX_COUNT_DISTINCT 牺牲了少量的精确度，却换来了计算效率的极大提升。

语法

APPROX_COUNT_DISTINCT(expr)

参数

返回类型

返回 NUMBER 类型数据。

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;

执行以下语句：

SELECT last_name, salary, APPROX_COUNT_DISTINCT(salary) OVER (ORDER BY hiredate) "Variance"
FROM employees WHERE manager_id = 100 ORDER BY last_name, salary, "Variance";

查询结果如下：

+-----------+--------+----------+
| LAST_NAME | SALARY | Variance |
+-----------+--------+----------+
| De Haan   |  11000 |        3 |
| Errazuriz |   1400 |        2 |
| Hartstein |  14000 |        4 |
| Partners  |  14000 |        4 |
| Raphaely  |   1700 |        1 |
| Raphaely  |   1700 |        2 |
| Russell   |  13000 |        6 |
| Weiss     |  13500 |        5 |
+-----------+--------+----------+