SQL Server 中Inner join 和where的效率差異
總結出來時說：對小數據量（<N萬）的來說效率幾乎無差異，更有說法說Inner join 和Where只是SQL標准不同，在查詢分析器中SQL Server查詢分析器是將Where直接轉換為Join后查詢的。
如是有了如下比較結果（均在查詢分析器中查詢和計時）：
語句（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
查詢結果，74行，共時間0：00：04
語句（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，時間0：00：01
后檢查發現語句（1）中有一個重復自查詢條件 ：customer_operator.operatorId = customer_operator.operatorId
將其葉加到語句2中，語句（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
所用時間和結果都為74行，時間0：00：01。
將語句（1）中的去掉該條件后成為語句（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
時間和結果為74行，時間0：00：01。
終於發現了些他們的差異。
結論：
盡量使用Join 而不是Where來列出關聯條件，特別是多個表聯合的時候。
原因是：
（1）在效率上，Where可能具有和Inner join一樣的效率。但基本可以肯定的（通過SQLServer幫助和其它資料，以及本測試）是Join的效率不比Where差。
（2）使用Join可以幫助檢查語句中的無效或者誤寫的關聯條件