1. SELECT employee_id, last_name
    2. FROM employees
    3. WHERE salary =
    4. (SELECT MIN(salary)
    5. FROM employees
    6. GROUP BY department_id);

    错误原因:

    Subquery returns more than 1 row

    子查询的结果有多行,不能直接用等于号。应该使用多行子查询的操作符:in