WITH AS短语，也叫做子查询部分，定义一个SQL片断后，该SQL片断可以被整个SQL语句所用到。有的时候，with as是为了提高SQL语句的可读性，减少嵌套冗余。

with A as (
  select  * 
  from user
) 
select * 
from A, customer 
where 
  customer.userid = user.id**

先执行select * from user把结果放到一个临时表A中，作为全局使用。

with as的用法可以通俗点讲是，讲需要频繁执行的slq片段加个别名放到全局中，后面直接调用就可以，这样减少调用次数，优化执行效率。

语法：

针对一个别名

with tmp as (select * from tb_name)

针对多个别名

with

tmp as (select * from tb_name),

tmp2 as (select * from tb_name2),

tmp3 as (select * from tb_name3),

…

例子：

–相当于建了个e临时表

with e as (select * from scott.emp e where e.empno=7499)

select * from e;

–相当于建了e、d临时表

with

 e as (select * from scott.emp),  

 d as (select * from scott.dept)  

select * from e, d where e.deptno = d.deptno;

与UNION ALL结合使用

with

sql1 as (select to_char(a) s_name from test_tempa),  

sql2 as (select to_char(b) s_name from test_tempb where not exists (select s_name from sql1 where rownum=1))  

select * from sql1

union all

select * from sql2

union all

select ‘no records’ from dual

   where not exists (select s_name from sql1 where rownum=1)  

   and not exists (select s_name from sql2 where rownum=1); 

WITH语句的优点:

1、可以轻松构建一个临时表，通过对这个表数据进行再处理。但是他比临时表更强大，临时表在会话结束才会自动被P清除，但with as临时表查询完成后就被清除了

2、复杂的查询会产生很大的sql，with as语法可以把一些公共查询提出来，也可以作为一个中间结果，可以使整个sql语句显得有条理些，提高可读性