查找所有员工的last_name和first_name以及对应的dept_name

最直观的LEFT JOIN和JOIN的区别

三表连接

SELECT e.last_name, e.first_name, d.dept_name
FROM employees e LEFT JOIN (SELECT emp_no, dept_name 
                   FROM dept_emp LEFT JOIN departments
                  ON dept_emp.dept_no = departments.dept_no) d
ON e.emp_no = d.emp_no