26_ mysql数据操作语言：DELETE语句

-- DELETE语句

-- 删除10部门中，工龄超过20年的员工记录

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) AS avg FROM t_emp GROUP BY deptno) t ON e.deptno=t.deptno AND e.sal<t.avg;

-- 删除员工King和他的直接下属的员工记录，用表连接实现

DELETE e
FROM t_emp e JOIN (SELECT empno FROM t_emp WHERE ename="KING") t
ON e.mgr=t.empno OR e.empno=t.empno;

TRUNCATE TABLE t_emp;