WITH AS用法

单个临时表语法格式：with tmp as (select * from tb_name)

多个临时表的语法：

with
tmp as (select * from tb_name),
tmp2 as (select * from tb_name2),
tmp3 as (select * from tb_name3)

作用：

(1)SQL可读性增强，更有层次感。

(2)with子查询只执行一次，将结果存储在用户临时表空间中，可以引用多次，增强性能。

(3)将几个临时表建立关联

例子：