mysql多列子查询

SELECT 
    d.department_name,
    (
        SELECT COUNT(*) 
        FROM employees e 
        WHERE e.department_id = d.department_id
    ) AS employee_count,
    (
        SELECT AVG(salary) 
        FROM employees e 
        WHERE e.department_id = d.department_id
    ) AS average_salary
FROM 
    departments d;

SELECT 
    d.department_name,
    COUNT(e.employee_id) AS employee_count,
    AVG(e.salary) AS average_salary
FROM 
    departments d
LEFT JOIN 
    employees e ON d.department_id = e.department_id
GROUP BY 
    d.department_id;

SELECT 
    d.department_name
FROM 
    departments d
WHERE 
    EXISTS (
        SELECT 1 
        FROM employees e 
        WHERE e.department_id = d.department_id
    );