Apply简单使用举例
APPLY

APPLY运算符把右表表达式应用左输入中的每一行。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中表达式。这一步会把左边每一行和来自右表表达式的相应行进行匹配，并将生成的结果集合并起来，返回组合后结果。

APPLY有两种类型：
1、CROSS APPLY 2、OUTER APPLY
两者的区别是如果内部（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

举例 :
创建一个顾客 表和一个订单员

SET NOCOUNT ON;IF OBJECT_ID('dbo.Orders') IS NOT NULL DROP TABLE dbo.Orders;IF OBJECT_ID('dbo.Customers') IS NOT NULL DROP TABLE dbo.Customers;GOCREATE TABLE dbo.Customers(  customerid  CHAR(5)     NOT NULL PRIMARY KEY,  city        VARCHAR(10) NOT NULL);CREATE TABLE dbo.Orders(  orderid    INT     NOT NULL PRIMARY KEY,  customerid CHAR(5)     NULL REFERENCES Customers(customerid));GOINSERT INTO dbo.Customers(customerid, city) VALUES('FISSA', 'Madrid');INSERT INTO dbo.Customers(customerid, city) VALUES('FRNDO', 'Madrid');INSERT INTO dbo.Customers(customerid, city) VALUES('KRLOS', 'Madrid');INSERT INTO dbo.Customers(customerid, city) VALUES('MRPHS', 'Zion');INSERT INTO dbo.Orders(orderid, customerid) VALUES(1, 'FRNDO');INSERT INTO dbo.Orders(orderid, customerid) VALUES(2, 'FRNDO');INSERT INTO dbo.Orders(orderid, customerid) VALUES(3, 'KRLOS');INSERT INTO dbo.Orders(orderid, customerid) VALUES(4, 'KRLOS');INSERT INTO dbo.Orders(orderid, customerid) VALUES(5, 'KRLOS');INSERT INTO dbo.Orders(orderid, customerid) VALUES(6, 'MRPHS');INSERT INTO dbo.Orders(orderid, customerid) VALUES(7, NULL);SELECT * FROM dbo.Customers;SELECT * FROM dbo.Orders;   --要求查询为每个顾户返回具有最大订单ID的两个订单-- Two most recent orders for each customerSELECT C.customerid, C.city, A.orderid FROM dbo.Customers AS C   CROSS APPLY     (SELECT TOP (2) O.orderid, O.customerid      FROM dbo.Orders AS O      WHERE O.customerid = C.customerid      ORDER BY orderid DESC) AS A;-- Two most recent orders for each customer,-- including customers that made no ordersSELECT C.customerid, C.city, A.orderidFROM dbo.Customers AS C  OUTER APPLY    (SELECT TOP (2) O.orderid, O.customerid     FROM dbo.Orders AS O     WHERE O.customerid = C.customerid     ORDER BY orderid DESC) AS A;