SQL Server 查询步骤

 (1)FROM <LEFT_TABLE> <JOIN_TYPE> JOIN <RIGHT_TABLE> ON <ON_PREDICATE>
         |<LEFT_TABLE> <APPLY_TYPE> APPLY <RIGHT_TABLE_EXPRESSION> AS <alias>
         |<LEFT_TABLE> pivot(<pivot_specification>) AS <alias>
         |<LEFT_TABLE> UNPIVOT(<unpivot_specification>) AS <alias>
 (2)WHERE<where_predicate>
 (3)GROUP BY<group_by_specification>
 (4)HAVING<having_predicate>
 (5)SELECT <DISTINCT> <TOP> <select_list>
 (6)ORDER BY<order_by_list>

--创建测试表

--创建顾客表
CREATE TABLE Customers
(custid INT NOT NULL PRIMARY KEY,
city NVARCHAR(20) NOT NULL
)
go
INSERT INTO Customers VALUES(1,‘深圳‘),(2,‘广州‘),(3,‘武汉‘),(4,‘上海‘),(5,‘北京‘)

--创建订单表
CREATE TABLE Orders
(orderid INT NOT NULL PRIMARY KEY IDENTITY(1000,1),
custid INT NOT NULL,
orderdate DATETIME NOT NULL
)
GO
INSERT INTO Orders(custid,orderdate)values(1,‘2013-10-1 00:00:00‘),(1,‘2013-10-2 00:00:00‘),(1,‘2013-10-3 00:00:00‘),(1,‘2013-10-4 00:00:00‘),(2,‘2013-10-1 00:00:00‘),(2,‘2013-10-3 00:00:00‘),(2,‘2013-10-5 00:00:00‘),(3,‘2013-10-3 00:00:00‘),(3,‘2013-10-7 00:00:00‘),(4,‘2013-10-1 00:00:00‘)

--创建订单明细表
CREATE TABLE [OrderDetails](
    [orderid] [int] NOT NULL,
    [productid] [int] NOT NULL,
    [unitprice] [money] NOT NULL,
    [qty] [smallint] NOT NULL
 CONSTRAINT [PK_OrderDetails] PRIMARY KEY CLUSTERED 
(
    [orderid] ASC,
    [productid] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
INSERT INTO OrderDetails VALUES(1000,10,5.00,1),(1000,14,6.00,2),(1001,10,5.31,3),(1001,11,5.22,1),(1001,12,3.20,3),(1001,13,4.10,2),(1002,11,7.00,1),(1003,12,8.00,5),(1004,13,8.41,1),(1004,11,6.65,1),(1005,18,7.41,1),(1006,17,10.00,1)

--查询深圳、广州每一个顾客每笔金额大于10的订单,并按订单价格倒序排序
SELECT TA.custid,TB.orderid,SUM(tc.unitprice*tc.qty) AS price FROM Customers TA LEFT JOIN Orders TB ON TA.custid=TB.custid LEFT JOIN OrderDetails TC ON TB.orderid=tc.orderid
WHERE TA.city IN(‘深圳‘,‘广州‘)
GROUP BY TA.custid,TB.orderid
HAVING SUM(tc.unitprice*tc.qty)>10
ORDER BY price DESC

对于上面的查询例子：FROM Customers TA LEFT JOIN Orders TB ON TA.custid=TB.custid的左连接的分解是这样的

第一步交叉连接、SELECT * FROM Customers TA CROSS JOIN Orders TB---首先进行交叉连接得到的行数是5*10=50行

第二步ON筛选、将TA.custid=TB.custid以外的结果排除,可以等价于SELECT * FROM Customers TA CROSS JOIN Orders TB WHERE TA.custid=TB.custid
第三步、将主表(左边的表)不在第二步的行加上,可以等价于 SELECT * FROM Customers TA CROSS JOIN Orders TB WHERE TA.custid=TB.custid union all SELECT * FROM Customers TA LEFT JOIN Orders TB ON TA.custid=TB.custid WHERE TB.custid IS NULL

 所以其它几个表运算只要大家知道怎么使用就可以了，大家只要明白它在T-SQL语句中的位置就行。

这里要注意一点：大家理解了JEFT JOIN的原理之后就明白"on"筛选对查询的删除不是最终的，在上面的第三步会把主表的一些行又添加上来，所以我们有时候写LEFT JOIN的时候有的人不太明白为什么ON 后面加AND和把AND放在WHERE里面的得到的结果不一样，就是这个原理了，WHERE操作对查询的删除才是最终的。

---当查询没有排序时
SELECT * FROM Orders

---当查询有排序时
SELECT * FROM Orders
ORDER BY CUSTID

对于一个没有外部查询的语句，order by 操作既能保证结果根据制定条件的排序，又能满足TOP的逻辑运算（查询最小的三个orderid）
SELECT TOP (3) * FROM Orders
ORDER BY ORDERID

对于存在外部查询时,order by在作用仅仅是保证top的逻辑结果的正确输出，而不能保证查询结果的排序，虽然我们可能查询出的结果是按照这个方式排序。
---当不指定TOP时报错
SELECT * FROM(SELECT  custid,orderid,orderdate FROM Orders ORDER BY orderdate DESC) AS A
---当指定
SELECT * FROM(SELECT TOP (3) custid,orderid,orderdate FROM Orders ORDER BY orderdate DESC) AS A