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;
GO

CREATE 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)
);
GO

INSERT 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 customer
SELECT 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 orders
SELECT C.customerid, C.city, A.orderid
FROM 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;