elect ename,job,sal
from emp where deptno>10
order by sal desc;
联合查询，PK dept.deptno FK emp.deptno
select emp.ename,dept.dname,dept.deptno from emp join dept on emp.deptno = dept.deptno;
也可以指定别名：
select e.ename,d.dname,d.deptno from emp e join dept d on e.deptno = d.deptno;
select e.ename,d.dname,d.deptno from emp e join dept d on e.deptno = d.deptno where e.ename='SMITH';
dept : 主键被参照，是主表，父表
emp： 外键参照dept的PK，从表
select e.ename,d.dname,d.deptno from emp e join dept d on e.deptno = d.deptno where e.ename='SMITH';
emp
驱动表
dept 匹配表
1. 匹配不上， t1表的该结果不会出现
2. t1 在t2中只有一个匹配
3. t1在t2有多条记录， 这时会形成多个组合，出现在结果集里面
不管驱动表还是匹配表，只有匹配之后才会有记录
select e.ename,e.sal,d.dname from emp e join dept d on e.deptno = d.deptno AND e.job='MANAGER';
非等值连接
select e.ename,e.sal, s.grade
from emp e join salgrade s
on e.sal between s.losal and s.hisal;
自参照 自己的某一列参照自己的PK
select worker.ename as name, manager.ename as Mname
from emp worker join emp manager on worker.mgr = manager.empno;
内连接 等值连接，非等值 自连接
外连接
select e.ename,d.dname
from emp e join dept d
on e.deptno = d.deptno;
select e.ename,d.dname
from emp e left outer join dept d
on e.deptno = d.deptno;
left outer
t1度一条数据。T2中开始匹配，找得到就形成记录，匹配不上，那么，就放一个null
t1 left outer t2 t1 是驱动表
t1 right outer t2 t2是驱动表
t1 full outer t2 无论是否匹配，都将显示
select e.ename, d.dname
from emp e full outer join dept d
on e.deptno = d.deptno;