SQL的view(视图)

create view v as < query expression >

0. 理解：view 可以看成是一个虚拟的表，这个表有以下几个特征

   保存的仅仅是代码
   不像 table 一样占用磁盘空间

1. 创建
   Once a view is defined, the view name can be used to refer to the virtual relation that the view generates
   When a view is created, the query expression is stored in the database
   
2. 操作