SQL

JOIN : left join、right join 、 inner join

inner join : 該查詢將返回左表（表A）中所有與右表（表B）中具有匹配記錄的記錄。

left join : 無論這些記錄與右表（表B）中是否匹配，查詢都返回左表（表A）中的所有記錄及與（表B）匹配的記錄。

right join: 無論這些記錄與左表（表A）中是否匹配，查詢都返回右表（表B）中的所有記錄及與（表A）匹配的記錄。

sql 指令 (補充說明)

select * from EMP cross join DEPT;

select * from emp natural join dept;

select e.ename,d.dname from emp e join dept d on e.deptno = d.deptno;

select e.ename,e.sal,s1.level as leve
from emp e join SAL_LEVEL s1 on e.sal between s1.SAL_MIN and s1.SAL_MAX
order by e.sal;

select * from emp where (job,mgr) in (select job,mgr from emp where empno =7499);

select * from emp where (job,sal) in (select job,sal from emp group by job,sal having count(*)>1 );