如何查询没有员工的部门？三种SQL实现方式 - 数据库优化技巧

实现方式1：使用子查询

SELECT department_name FROM departments WHERE department_id NOT IN (SELECT department_id FROM employees)

实现方式2：使用LEFT JOIN

SELECT departments.department_name FROM departments LEFT JOIN employees ON departments.department_id = employees.department_id WHERE employees.employee_id IS NULL

实现方式3：使用NOT EXISTS

SELECT department_name FROM departments WHERE NOT EXISTS (SELECT * FROM employees WHERE departments.department_id = employees.department_id)