where 子句的执行顺序
<p>看代码：</p>
<p class="codetitle">
<span><u>复制代码</u></span> 代码如下:</p>
<p class="codebody">
<br>
set statistics io on<br>
set statistics time on<br>
go<br>
set statistics profile on<br>
go<br><br>
use pubs<br>
select * from authors<br>
where (au_fname like 'S%' or au_fname like 'a%')<br>
and (state like 'CA' or state like 'UT')<br>
and (cast(zip as int) > 90000)<br><br>
select * from authors<br>
where (au_fname like 'S%' or au_fname like 'a%')<br>
and (cast(zip as int) > 90000)<br>
and (state like 'CA' or state like 'UT')<br><br>
select * from authors<br>
where (cast(zip as int) > 90000)<br>
and (au_fname like 'S%' or au_fname like 'a%')<br>
and (state like 'CA' or state like 'UT')</p>
<p>
<br>
目的：想验证where语句的各个条件的查询顺序<br>
环境：SQLServer 2005Express版本<br>
步骤：显示查询计划<br>
结果：无一例外，都被统一成了这样的代码</p>
<p class="codetitle">
<span><u>复制代码</u></span> 代码如下:</p>
<p class="codebody">
<br>
|--Clustered Index Scan(OBJECT:(...), WHERE:(CONVERT(int,...,0)>(90000) AND (... like 'S%' OR ... like 'a%') AND (... like 'CA' OR ... like 'UT')))</p>
<p>
<br>
结论：貌似在2005之前的版本中，where子句的顺序是从前往后的。但是又貌似在2005的版本中进行了优化，所有的顺序都被统一成了以过滤能力进行排序的语句。</p>
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