加入两个SQL查询

-- employees 表
| employee_id | name   | department_id |
|-------------|--------|---------------|
| 1           | Alice  | 10            |
| 2           | Bob    | 20            |
| 3           | Carol  | 10            |

-- departments 表
| department_id | department_name |
|---------------|-----------------|
| 10            | HR              |
| 20            | IT              |

SELECT department_id, COUNT(*) as employee_count
FROM employees
GROUP BY department_id;

SELECT department_id, department_name
FROM departments;

SELECT d.department_name, e.employee_count
FROM (
    SELECT department_id, COUNT(*) as employee_count
    FROM employees
    GROUP BY department_id
) e
JOIN departments d
ON e.department_id = d.department_id;

| department_name | employee_count |
|-----------------|----------------|
| HR              | 2              |
| IT              | 1              |