交叉应用有时可以让您做一些内部连接无法做到的事情。

示例(语法错误):

select F.* from sys.objects O  
inner join dbo.myTableFun(O.name) F   
on F.schema_id= O.schema_id

这是一个语法错误，因为当与内部连接一起使用时，表函数只能接受变量或常量作为参数。(也就是说，表函数参数不能依赖于另一个表的列。)

然而:

select F.* from sys.objects O  
cross apply ( select * from dbo.myTableFun(O.name) ) F  
where F.schema_id= O.schema_id

这是合法的。

编辑: 或者，更短的语法:(通过ErikE)

select F.* from sys.objects O  
cross apply dbo.myTableFun(O.name) F
where F.schema_id= O.schema_id

编辑:

注意: Informix 12.10 xC2+有横向派生表，Postgresql(9.3+)有横向子查询，可以达到类似的效果。

在我看来，CROSS APPLY可以在复杂/嵌套查询中处理计算字段时填补一定的空白，并使它们更简单，更易于阅读。

简单的例子:你有一个DoB，你想要显示多个与年龄相关的字段，这些字段也依赖于其他数据源(比如就业)，比如Age、AgeGroup、AgeAtHiring、MinimumRetirementDate等，以便在最终用户应用程序中使用(例如Excel数据透视表)。

选择是有限的，很少是优雅的:

JOIN subqueries cannot introduce new values in the dataset based on data in the parent query (it must stand on its own). UDFs are neat, but slow as they tend to prevent parallel operations. And being a separate entity can be a good (less code) or a bad (where is the code) thing. Junction tables. Sometimes they can work, but soon enough you're joining subqueries with tons of UNIONs. Big mess. Create yet another single-purpose view, assuming your calculations don't require data obtained mid-way through your main query. Intermediary tables. Yes... that usually works, and often a good option as they can be indexed and fast, but performance can also drop due to to UPDATE statements not being parallel and not allowing to cascade formulas (reuse results) to update several fields within the same statement. And sometimes you'd just prefer to do things in one pass. Nesting queries. Yes at any point you can put parenthesis on your entire query and use it as a subquery upon which you can manipulate source data and calculated fields alike. But you can only do this so much before it gets ugly. Very ugly. Repeating code. What is the greatest value of 3 long (CASE...ELSE...END) statements? That's gonna be readable! Tell your clients to calculate the damn things themselves.

我错过什么了吗?可能吧，请随意评论。但是，嘿，CROSS APPLY在这种情况下就像天赐之物:您只需添加一个简单的CROSS APPLY(选择tbl。value + 1 as someFormula)作为crossTbl和voilà!您的新字段现在已经可以使用了，就像它一直在源数据中一样。

通过CROSS APPLY引入的值可以…

用于创建一个或多个计算字段，而不会增加性能、复杂性或可读性问题 像join一样，几个后续的CROSS APPLY语句可以引用它们自己:someFormula + 1 as someMoreFormula) as crossTbl2 您可以在后续的JOIN条件中使用CROSS APPLY引入的值 另外，还有表值函数方面

该死，没有什么是他们做不到的!

好吧，我不确定这是否有资格作为使用交叉应用与内部连接的原因，但这个查询在论坛帖子中使用交叉应用为我回答了，所以我不确定是否有一个等效的方法使用内部连接:

Create PROCEDURE [dbo].[Message_FindHighestMatches]

-- Declare the Topical Neighborhood
@TopicalNeighborhood nchar(255)

作为 开始

-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON

Create table  #temp
(
    MessageID         int,
    Subjects          nchar(255),
    SubjectsCount    int
)

Insert into #temp Select MessageID, Subjects, SubjectsCount From Message

Select Top 20 MessageID, Subjects, SubjectsCount,
    (t.cnt * 100)/t3.inputvalues as MatchPercentage

From #temp 

cross apply (select count(*) as cnt from dbo.Split(Subjects,',') as t1
             join dbo.Split(@TopicalNeighborhood,',') as t2
             on t1.value = t2.value) as t
cross apply (select count(*) as inputValues from dbo.Split(@TopicalNeighborhood,',')) as t3

Order By MatchPercentage desc

drop table #temp

END

交叉应用有时可以让您做一些内部连接无法做到的事情。

示例(语法错误):

select F.* from sys.objects O  
inner join dbo.myTableFun(O.name) F   
on F.schema_id= O.schema_id

这是一个语法错误，因为当与内部连接一起使用时，表函数只能接受变量或常量作为参数。(也就是说，表函数参数不能依赖于另一个表的列。)

然而:

select F.* from sys.objects O  
cross apply ( select * from dbo.myTableFun(O.name) ) F  
where F.schema_id= O.schema_id

这是合法的。

编辑: 或者，更短的语法:(通过ErikE)

select F.* from sys.objects O  
cross apply dbo.myTableFun(O.name) F
where F.schema_id= O.schema_id

编辑:

注意: Informix 12.10 xC2+有横向派生表，Postgresql(9.3+)有横向子查询，可以达到类似的效果。

当你需要子查询的列时，交叉应用可以用来替换子查询

子查询

select * from person p where
p.companyId in(select c.companyId from company c where c.companyname like '%yyy%')

在这里，我将无法选择公司表的列 使用交叉应用

select P.*,T.CompanyName
from Person p
cross apply (
    select *
    from Company C
    where p.companyid = c.companyId and c.CompanyName like '%yyy%'
) T

这个问题在技术上已经得到了很好的回答，但让我举一个具体的例子来说明它是如何非常有用的:

假设您有两个表，Customer和Order。客户有很多订单。

我想创建一个视图，提供关于客户的详细信息，以及他们最近下的订单。对于仅使用join，这将需要一些自连接和聚合，这并不漂亮。但交叉应用，它超级简单:

SELECT *
FROM Customer
CROSS APPLY (
  SELECT TOP 1 *
  FROM Order
  WHERE Order.CustomerId = Customer.CustomerId
  ORDER BY OrderDate DESC
) T