-- 内部結合（INNER JOIN）: 部署が割り当てられた社員のみ表示
SELECT e.employee_id, e.first_name, d.dept_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.dept_id;
-- 左外部結合（LEFT OUTER JOIN）: 部署未割り当ての社員も表示
SELECT e.first_name, d.dept_name
FROM employees e
LEFT OUTER JOIN departments d ON e.dept_id = d.dept_id;