mysql delete

# 删除10部门中 工龄超过20年的员工
delete from t_emp
where deptno = 10 and DATEDIFF(NOW(),hiredate)/365 >= 20

#删除20部门中工资最高的员工记录
delete from t_emp
where deptno = 20
ORDER BY sal + IFNULL(comm,0) DESC
limit 1

# 删除 SALES 部门和该部门的全部员工记录
DELETE e,d
from t_emp e join t_dept d
on e.deptno = d.deptno
where d.dname = 'SALES'

# 删除每个低于部门平均底薪的员工记录
delete e from t_emp e
join (
select deptno, avg(sal + IFNULL(comm,0)) sal from t_emp
GROUP BY deptno) as t
on e.deptno = e.deptno and e.sal < t.sal

# 删除员工king 和他的直接下属员工记录
delete e from t_emp e
join
(select empno from t_emp where ename = 'KING') as t on t.empno = e.mgr or e.empno = t.empno

# 删除SALES部门的员工,以及没有部门的员工 

delete e
from t_emp e
left join t_dept d on e.deptno = d.deptno
where d.dname = 'SALES' or e.deptno is null

清空数据表数据

 TRUNCATE TABLE    t_emp