SQL Server 中Inner join 和where的效率差异
今天,手头上正在作的一个项目,在生成报表时,客户感觉太慢,于是,各处检查,看可否提示效率。界面上的都改进了,提升不大。如是在SQL 语句上下功夫。(我这人比较懒,对简单的语句和查询都没有经过仔细优化的,一般只对姚使用left join,outer join,group by 以及carsor的语句会仔细写并用数据库理论考虑和检查---因为这种语句一般测试时如果发现错误,检查和调试很麻烦)
先在网上Google搜索“Join 与 Where 效率”以及察看SQL Server 帮助文档,希望能获得“捷径”些的优化思路。
搜索的结果是,各大论坛,包括MSDN上很多人提出了这个问题,但回答是众说纷纭。总体上总结出来时说:对小数据量(<N万)的来说效率几乎无差异,更有说法说Inner join 和Where只是SQL标准不同,在查询分析器中SQL Server查询分析器是将Where直接转换为Join后查询的。
还是自己来做试验吧。
如是有了如下比较结果(均在查询分析器中查询和计时):
语句(1)declare @operatorName nvarchar(50)set @operatorName = '%'
select distinct item.*; from item , customer_item , customer_operator ,operatorwhere item.itemcode = customer_item.itemCode and customer_item.customerCode =; customer_operator.customerCodeand customer_operator.operatorId =; customer_operator.operatorIdand operator.operatorName like @operatorNameand item.deleted = 0 and customer_item.deleted = 0 and customer_operator.deleted = 0查询结果,74行,共时间0:00:04
语句(2)declare @operatorName nvarchar(50)
set @operatorName = '%'
select distinct item.*; from item inner join; customer_item on; item.itemcode = customer_item.itemCode inner join customer_operator on customer_item.customerCode = customer_operator.customerCodeinner join operator on customer_operator.operatorId = operator.operatorIdwhere; operator.operatorName like @operatorNameand item.deleted = 0 and customer_item.deleted = 0 and customer_operator.deleted = 0共74行,时间0:00:01
后检查发现语句(1)中有一个重复自查询条件 :customer_operator.operatorId =; customer_operator.operatorId将其叶加到语句2中,语句(3)declare @operatorName nvarchar(50)
set @operatorName = '%'
select distinct item.*; from item inner join; customer_item on; item.itemcode = customer_item.itemCode inner join customer_operator on customer_item.customerCode = customer_operator.customerCodeinner join operator on customer_operator.operatorId = operator.operatorIdwhere; operator.operatorName like @operatorNameand item.deleted = 0 and customer_item.deleted = 0 and customer_operator.deleted = 0and customer_operator.operatorId =; customer_operator.operatorId
所用时间和结果都为74行,时间0:00:01。
将语句(1)中的去掉该条件后成为语句(4)declare @operatorName nvarchar(50)set @operatorName = '%'
select distinct item.*; from item , customer_item , customer_operator ,operatorwhere item.itemcode = customer_item.itemCode and customer_item.customerCode =; customer_operator.customerCode--and customer_operator.operatorId =; customer_operator.operatorIdand operator.operatorName like @operatorNameand item.deleted = 0 and customer_item.deleted = 0 and customer_operator.deleted = 0
时间和结果为74行,时间0:00:01。
终于发现了些他们的差异。
结论: 尽量使用Join 而不是Where来列出关联条件,特别是多个表联合的时候。原因是: (1)在效率上,Where可能具有和Inner join一样的效率。但基本可以肯定的(通过SQLServer帮助和其它资料,以及本测试)是Join的效率不比Where差。 (2)使用Join可以帮助检查语句中的无效或者误写的关联条件