count和group by在包含3个空值的表的层次结构中

CREATE TABLE employees (
    id INT PRIMARY KEY,
    department VARCHAR(50),
    position VARCHAR(50),
    salary DECIMAL(10, 2)
);

INSERT INTO employees (id, department, position, salary) VALUES
(1, 'HR', 'Manager', 50000),
(2, 'HR', 'Assistant', 30000),
(3, NULL, 'Developer', 60000),
(4, 'IT', NULL, 70000),
(5, NULL, NULL, 80000);

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

department | employee_count
-----------|----------------
HR         | 2
IT         | 1
NULL       | 2

SELECT position, COUNT(*) AS employee_count
FROM employees
GROUP BY position;

position   | employee_count
-----------|----------------
Assistant  | 1
Developer  | 1
Manager    | 1
NULL       | 2

SELECT COALESCE(department, 'Unknown') AS department, COUNT(*) AS employee_count
FROM employees
GROUP BY COALESCE(department, 'Unknown');

department | employee_count
-----------|----------------
HR         | 2
IT         | 1
Unknown    | 2

SELECT department, COUNT(*) AS employee_count
FROM employees
WHERE department IS NOT NULL
GROUP BY department;

department | employee_count
-----------|----------------
HR         | 2
IT         | 1