为什么要用WITH？
1. 如果需要在一段复杂查询里多次应用同一个查询，用WITH可实现代码重用；
2. WITH查询类似将查询结果保留到用户临时表里，在大的复杂查询中可以减少IO，有一定的性能优化作用。
WITH查询有何限制与特性？
1. 如果当前schema下有与WITH查询别名相同的表，查询中WITH查询生成的表优先；
2. 只能用于select 语句；
3. WITH可包含一个或多个查询；
4. WITH查询可被其它查询或WITH查询引用。
示例：
duzz$scott@orcl>select * from dept;
DEPTNO DNAME LOC
---------- --------------- ----------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
Elapsed: 00:00:00.00
duzz$scott@orcl>with dept as (select 1 a from dual) select * from dept;
A
----------
1
Elapsed: 00:00:00.00
duzz$scott@orcl>with dept as (select 1 a from dual) delete from dept where a=1;
with dept as (select 1 a from dual) delete from dept where a=1
*
ERROR at line 1:
ORA-00928: missing SELECT keyword
Elapsed: 00:00:00.01
duzz$scott@orcl>with wt1 as (select 1 a, 2 b from dual), wt2 as (select 1 c,3 d from dual) select * from wt1,wt2 where wt1.a=wt2.c;
A B C D
---------- ---------- ---------- ----------
1 2 1 3
Elapsed: 00:00:00.00
duzz$scott@orcl>with wt1 as (select 10 a, 2 b from dual), wt2 as (select deptno,loc from dept,wt1 where deptno=a) select loc from wt2;
LOC
---------------------------------------
NEW YORK
Elapsed: 00:00:00.00
duzz$scott@orcl>
