視圖

1. 創建視圖：view

   視圖：只是提供了一個查詢的窗口，本身不存儲數據，所有數據都來源於原表

   創建視圖
       create view emp01 as select e.ename,e.sal from emp e;
   創建只讀視圖:with read only
       create view emp02 as select e.ename e.sal from emp e;

   查詢視圖
       select * from emp01;

   修改視圖(不推薦,會直接修改原表中的數據)
       update emp01 set job = 'CLERK' where ename = 'ALLEN';
       commit;