create view v as < query expression >
- 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 - 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
- 操作
