Oracle PL/SQL之WITH查询

2023-07-07 22:54•杂谈•阅读 3308

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>

上一篇 »Oracle数据库之PL/SQL异常处理
下一篇 »oracle pl/sql之sql中的case语句

Oracle PL/SQL之WITH查询

相关推荐

Oracle PL/SQL之GROUP BY GROUPING SETS

Oracle PL/SQL之GROUP BY CUBE

oracle pl/sql之使用游标变量更新数据库

简单搞一下 Oracle 存储过程动态SQL之获取查询总数！

ORACLE HANDBOOK系列之四：批量SQL，BULK SQL

Oracle数据库之PL/SQL触发器

oracle学习笔记，十八 PL/SQL 游标

Oracle PL/SQL高级应用 游标

Oracle PL/SQL高级应用游标