Oracle 自连接 / 外连接 / 子查询

--连接查询的三种格式
select ename, deptno,dname from emp natural join dept;
select ename, deptno,dname from emp join dept using(deptno);
select emp.ename, emp.deptno,dept.dname from emp join dept on emp.deptno!=dept.deptno;

--自连接，将一张表当成两张表来看待
select e.ename,m.ename from emp e join emp m on e.mgr= m.empno

--外连接right outer join/Left outer join/full outer join
select ename,d.deptno,d.dname
from emp right outer join dept d on emp.deptno= d.deptno

--连接更多表
 select ename,dname,grade
 from emp join dept on emp.deptno=dept.deptno
          join salgrade on sal BETWEEN losal and hisal;

---------------------------------------------------------------------子查询
--单行子查询：子查询返回一行
select * from emp where sal>(select avg(sal)from emp);

--多行子查询：子查询返回多行 ANY(任意) ALL(所有) IN(在里面)
select * from emp where sal>all(select avg(sal)from emp group by deptno);

--[not] exists 判断是否行存在
select ename,mgr
from emp e where EXISTS(select 1 from emp where e.empno=mgr);