Oracle PL/SQL之WITH查询

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>