APPROX_COUNT_DISTINCT

APPROX_COUNT_DISTINCT(expr)

obclient> 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;

obclient> 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";
+-----------+--------+----------+
| 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 |
+-----------+--------+----------+