1、显示工作20年以上的员工
SELECT * FROM employees.employees WHERE DATE_ADD(hire_date,INTERVAL 20 YEAR) < NOW()
2、显示包含五个字符的名字
SELECT first_name FROM employees.employees WHERE LENGTH(first_name) = 5
3、显示是否有没有雇佣时间的员工
SELECT * FROM employees.employees WHERE hire_date IS NULL
4、员工都是哪些姓，按姓排序正序排，显示员工姓名
SELECT DISTINCT(last_name) FROM employees.employees ORDER BY last_name
