SQL Server 中Inner join 和where的效率差异
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。
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 ,operator
where item.itemcode = customer_item.itemCode
and customer_item.customerCode = customer_operator.customerCode
and customer_operator.operatorId = customer_operator.operatorId
and operator.operatorName like @operatorName
and 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.customerCode
inner join operator on customer_operator.operatorId = operator.operatorId
where operator.operatorName like @operatorName
and 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.customerCode
inner join operator on customer_operator.operatorId = operator.operatorId
where operator.operatorName like @operatorName
and item.deleted = 0 and customer_item.deleted = 0 and customer_operator.deleted = 0
and 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 ,operator
where item.itemcode = customer_item.itemCode
and customer_item.customerCode = customer_operator.customerCode
--and customer_operator.operatorId = customer_operator.operatorId
and operator.operatorName like @operatorName
and 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