最近在论坛经常看到有人使用with table asobjects iis7站长之家，一般都是构建一个临时表，用于测试，经研究此语句的用法我理解有以下好处：
1） 不用实际建表，可以轻松构建一个临时表，通过对这个表的处理测试一些功能；
例如：with t as (
selectfrom dualunion all
selectfrom dualunion all
selectfrom dualunion all
selectfrom dualunion all
selectfrom dual)
select ||regexp_replace(replace(regexp_substr(tel,,1,1),,),,)from t;
2） 复杂的查询会产生很大的sql，with table as语法可以把一些公共查询提出来，也可以显示一个个中间结果，可以使整个sql语句显得有条理些，可读性提高；
3） 前面的中间结果可以被语句中的select或后面的中间结果表引用，类似于一个范围仅限于本语句的临时表，在需要多次查询某中间结果时可以提升效率 ，特别是对一些大数据量的表做多项统计时，可以大大提高效率。
例如：
with a as (select * from dba_objects where 某些查询条件),
b as (select * from a where 某些查询条件)
select * from b , a where 其它查询条件;
再比如：
with tb as (select * from dba_objects where 某些查询条件),
select count(*) from tb where 其它查询条件1
union
select count(*) from tb where 其它查询条件2
union
select count(*) from tb where 其它查询条件3;
1、with table as 相当于建个临时表（用于一个语句中某些中间结果放在临时表空间的SQL语句），Oracle 9i 新增WITH语法，可以将查询中的子查询命名，放到SELECT语句的最前面。
语法就是
with tempname as (select ....)
select ...
例子：
with t as (select * from emp where depno=10)
select * from t where empno=xxx
with wd as (select did,arg(salary) 平均工资 from work group by did),
em as (select emp.*,w.salary from emp left join work w on emp.eid = w.eid)
select * from wd,em where wd.did =em.did and wd.平均工资>em.salary;
2、何时被清除
临时表不都是会话结束就自动被PGA清除嘛! 但with as临时表是查询完成后就被清除了！
23:48:58 SCOTT@orcl> with aa as(select * from dept)
23:57:58 2 select * from aa;
DEPTNO DNAME LOC
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
已用时间: 00: 00: 00.12
23:58:06 SCOTT@orcl> select * from aa;
select * from aa
*
第 1 行出现错误:
ora-00942: 表或视图不存在
已用时间: 00: 00: 00.02
23:58:14 SCOTT@orcl>
3、举例
假定有张很大的表，有几年来的经营数据，数据量很大。如果要统计一段时间内的邮件状态，如果都从总表中统计，效率一定不高，而采用with tablename as 语句，先将一段时间内的数据取出来，再进行统计就会简单的多。
with tb as (
select b.city,a.mail_num,a.rcv_area from tb_evt_mail_clct a, tb_jg b
where a.clct_date = to_date('20110816', 'yyyymmdd')
and (a.rcv_area like '23%' or a.rcv_area like '24%')
and a.clct_bureau_org_code = b.zj_code
and not exists (select 1 from tb_evt_dlv c
where c.mail_num = a.mail_num
and c.dlv_sts_code = 'I')
) -- 提取出查询数据
select aa.city 收寄城市, aa.wtt 未妥投, bb.wtd 未投递, cc.wkc 未开拆
from (select tb.city, count(*) wtt
from tb
group by tb.city) aa -- 统计1
left join (select tb.city, count(*) wtd
from tb
where not exists
(select 1 from tb_evt_dlv c
where c.mail_num = tb.mail_num
and (c.dlv_sts_code = 'H' or c.dlv_sts_code = 'I'))
group by tb.city) bb on bb.city = aa.city -- 统计2
left join (select tb.city, count(*) wkc
from tb
where not exists
(select 1 from tb_evt_dlv c
where c.mail_num = tb.mail_num
and (c.dlv_sts_code = 'H' or c.dlv_sts_code = 'I'))
and not exists
(select 1 from tb_evt_bag_mail_rela e
where e.mail_num = tb.mail_num
and e.bag_actn_code = '2'
and e.deal_org_code like
substr(tb.rcv_area, 1, 4) || '%')
group by tb.city) cc on cc.city = aa.city -- 统计3