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;