/*
-- 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;
