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Oracle PL/SQL之WITH查询

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Release： 2016-06-07 17:04:38
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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

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：

linux

Related labels：
Oracle教程 Oracle PL/SQL之WITH查询
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