ＳＱＬ技术内幕－１

/*

--  逻辑查询处理的步骤序号



(5) SELECT   (5-2)DISTINCT  (5-2) TOP (top_specification)  (5-1)<select_list>

(1) FROM (1-J) <left_table>  <join-type> JOIN <right-table> ON <on_predicate>

     |(1-A) <left_table>  <apply-type> APPLY <right-table_expression> AS <alias>

	 |(1-P) <left_table>  PIVOT(<pivot_specification>) AS <alias>

	 |(1-U) <left_table>  UNPIVOT(<unpivot_specification>) AS <alias>

(2) WHERE	(where_precidate)

(3) GROUP BY  <group_by_specification>

(4) HAVING  <having_predicate>

(6) ORDER BY <order_by_list>	 

*/



/* 

每一步都会生成一个虚拟表，该虚拟表作为下一步的输入，这些虚拟表对于调用者是不可用的，

只有最后一步生成的虚拟表才会返回给调用者，如果在查询中没有指定某一个子句，最会跳过相应的步骤

*/

　　

SET NOCOUNT ON;

USE tempdb;



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;



-- Listing 1-2: Query: Madrid customers with Fewer than three orders



-- The query returns customers from Madrid who placed fewer than

-- three orders (including zero), and their order count.

-- The result is sorted by the order count.

SELECT C.customerid, COUNT(O.orderid) AS numorders

FROM dbo.Customers AS C

  LEFT OUTER JOIN dbo.Orders AS O

    ON C.customerid = O.customerid

WHERE C.city = 'Madrid'

GROUP BY C.customerid

HAVING COUNT(O.orderid) < 3

ORDER BY numorders;