sql-(Cross||Outer)Apply

Apply - 涉及以下两个步骤中的一步或两步（取决于Apply的类型）：
　　1、A1：把右表表达式应用于左表的行
　　2、A2：添加外部行
　　Apply运算符把右表表达式应用于左输入的每一行。右表达式可以引用左输入中的列，对于左表中的每一行，都要计算一次右边输入的表达式。这一步会把左边的每一行和来自右表达式的响应行进行匹配，并将生成的结果集合合并起来，返回组合后的结果
　　Cross Apply和Outer Apply总是包含步骤A1，只有Outer Apply才包含步骤A2
　　如果内部（右）表表达式为外部（左）行返回的是空集，则Cross Apply不会返回该外部（左）行。而Outer Apply会返回这样的行，对于内表表达式的属性，则使用Null作为其占位符。

If OBJECT_ID('Orders') Is Not Null Drop Table Orders;
If OBJECT_ID('Customers') Is Not Null Drop Table Customers;
Go

Create Table Customers
(
    CustomerID Char(5) Not Null Primary Key,
    City Varchar(10) Not Null
);

Create Table Orders
(
    OrderID Int Not Null Primary Key,
    CustomerID Char(5) Null References Customers(CustomerID)
);
Go

Insert Into Customers(CustomerID,City) Values('FISSA','Madrid');
Insert Into Customers(CustomerID,City) Values('FRNDO','Madrid');
Insert Into Customers(CustomerID,City) Values('KRLOS','Madrid');
Insert Into Customers(CustomerID,City) Values('MRPHS','Zion');

Insert Into Orders(OrderID,CustomerID) Values(1,'FRNDO');
Insert Into Orders(OrderID,CustomerID) Values(2,'FRNDO');
Insert Into Orders(OrderID,CustomerID) Values(3,'KRLOS');
Insert Into Orders(OrderID,CustomerID) Values(4,'KRLOS');
Insert Into Orders(OrderID,CustomerID) Values(5,'KRLOS');
Insert Into Orders(OrderID,CustomerID) Values(6,'MRPHS');
Insert Into Orders(OrderID,CustomerID) Values(7,Null);

Select * From Customers

Select * From Orders

Select  a.CustomerID,a.City,c.OrderID From Customers a
Cross Apply
(
    Select Top 1 * From Orders b
    Where a.CustomerID=b.CustomerID
    Order By OrderID Desc
) c

Select  a.CustomerID,a.City,c.OrderID From Customers a
Cross Apply
(
    Select Top 2 * From Orders b
    Where a.CustomerID=b.CustomerID
    Order By OrderID Desc
) c

Select  a.CustomerID,a.City,c.OrderID From Customers a
Outer Apply
(
    Select Top 1 * From Orders b
    Where a.CustomerID=b.CustomerID
    Order By OrderID Desc
) c