题解

# SELECT e.last_name, e.first_name, 
#     CASE
#         WHEN de.dept_no IS NULL THEN NULL
#         ELSE d.dept_name
#     END AS dept_name
# FROM employees e
# LEFT JOIN dept_emp de ON e.emp_no = de.emp_no
# LEFT JOIN departments d ON de.dept_no = d.dept_no;



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