出错样例

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
            SELECT manager_id
            FROM employees
);

查询结果为空，Why？
使用NOT IN并且子查询中含有 manager_id 为null的数据。
解决方案：
添加过滤条件。

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
            SELECT manager_id
            FROM employees
            WHERE manager_id IS NOT NULL
);