從這里下載文件employees.txt，customers.txt，orders.txt

參考文章：http://www.cnblogs.com/wwan/archive/2011/02/24/1964279.html

使用package導(dǎo)入數(shù)據(jù)：http://www.cnblogs.com/tylerdonet/archive/2011/04/17/2017471.html

簡單的聚合

從orders表中選擇各個(gè)年份共有共有多少客戶訂購了商品

•  第一種寫法，我們可以寫成這樣

  1select YEAR(o.orderdate) orderyear,COUNT(distinct(custid)) numCusts
  2from Sales.Orders o
  3group by YEAR(o.orderdate)
  4go

  要注意的是如果把group by YEAR(o.orderdata)換成group by orderyear就會(huì)出錯(cuò)，這里涉及到sql語句的執(zhí)行順序問題，有時(shí)間再了解一下　　　　　　　　　　
• 第二種寫法，

  1select orderyear,COUNT(distinct(custid))numCusts
  2from (select YEAR(orderdate) as orderyear,custid from sales.orders) as D
  3group by orderyear
  4go

  在from語句中先得到orderyear，然后再select語句中就不會(huì)出現(xiàn)沒有這個(gè)字段的錯(cuò)誤了
• 第三種寫法，

  1select orderyear,COUNT(distinct(custid)) numCusts
  2from (select YEAR(orderdate),custid from sales.orders) as D(orderyear,custid)
  3group by orderyear
  4go

  在as D后面加上選擇出的字段，是不是更加的清楚明了呢！
• 第四種寫法，with出場了

  1with c as(
  2select YEAR(orderdate) orderyear, custid from sales.orders)
  3select orderyear,COUNT(distinct(custid)) numCusts from c group by orderyear
  4go

  with可以使語句更加的經(jīng)湊，下面是權(quán)威解釋?！　?BR>　　　　
  指定臨時(shí)命名的結(jié)果集，這些結(jié)果集稱為公用表表達(dá)式 (CTE)。該表達(dá)式源自簡單查詢，并且在單條 SELECT、INSERT、UPDATE、MERGE 或 DELETE 語句的執(zhí)行范圍內(nèi)定義。該子句也可用在 CREATE VIEW 語句中，作為該語句的 SELECT 定義語句的一部分。公用表表達(dá)式可以包括對自身的引用。這種表達(dá)式稱為遞歸公用表達(dá)式?！　　　　　　　　　　　　　　?/EM>

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　----MSDN

• 第五種寫法，也可以借鑒第三種寫法，這樣使語句更加清楚明了，便于維護(hù)

  1 with c(orderyear,custid) as(
  2  select YEAR(orderdate),custid from sales.orders)
  3  select orderyear,COUNT(distinct(custid)) numCusts from c group by c.orderyear
  4  go

  上面5中寫法都得到相同的結(jié)果，如下圖1：圖1

添加計(jì)算

• 現(xiàn)在要求要求計(jì)算出訂單表中每年比上一年增加的客戶數(shù)目，這個(gè)稍微復(fù)雜

  1 with yearcount as(
  2  select YEAR(orderdate) orderyear,COUNT(distinct(custid)) numCusts from sales.orders group by YEAR(orderdate))
  3  select cur.orderyear curyear,cur.numCusts curNumCusts,PRv.orderyear prvyear,prv.numCusts prvNumCusts,cur.numCusts-prv.numCusts growth
  4  from yearcount cur left join yearcount prv on cur.orderyear=prv.orderyear+1
  5 go

  這里兩次使用到with結(jié)果集。查詢得到的結(jié)果如下圖2
  
  圖2

復(fù)雜的計(jì)算

• 查找客戶id，這些客戶和所有來自美國的雇員至少有一筆交易記錄，查詢語句如下
  

  1 with TheseEmployees as(
  2 select empid from hr.employees where country='USA'),
  3 CharacteristicFunctions as(
  4 select custid,
  5        case when custid in (select custid from sales.orders as o where o.empid=e.empid) then 1 else 0 end as charfun
  6 from sales.customers as c cross join TheseEmployees as e)
  7 select custid,min(charfun) from CharacteristicFunctions group by custid having min(charfun)=1
  8 go

  這里嵌套with語句，第with語句查找美國雇員的id，第二個(gè)語句使用這個(gè)結(jié)果和擁有客戶的客戶id和擁有關(guān)系標(biāo)識(shí)做笛卡爾積運(yùn)算。最后從這個(gè)笛卡爾積中通過標(biāo)識(shí)找到最終的custid。
  結(jié)果如下圖3
  
  圖3

這里只有簡單地介紹，沒有深入，高手們不要見笑啊。

---------------------------------------------------------分界線----------------------------------------------------------

with語句和子查詢的性能比較

 1 /*with查詢*/
 2 declare @withquery varchar(5000)
 3 declare @execcount int=0
 4 set @withquery='with TheseEmployees as(
 5 select empid from hr.employees where country=N''USA''),
 6 CharacteristicFunctions as(
 7 select custid,
 8        case when custid in (select custid from sales.orders as o where o.empid=e.empid) then 1 else 0 end as charfun
 9 from sales.customers as c cross join TheseEmployees as e)
10 select custid from CharacteristicFunctions group by custid having min(charfun)=1 order by custid
11 ' 
12 while @execcount<10
13 begin
14 exec (@withquery);
15 set @execcount=@execcount+1
16 end
17 
18 /*子查詢*/
19 declare @subquery varchar(5000)
20 declare @execcount int=0
21 set @subquery='select custid from Sales.Orders where empid in
22 (select empid from HR.Employees where country = N''USA'') group by custid
23 having count(distinct empid)=(select count(*) from HR.Employees where country = N''USA'');
24 '
25 while @execcount<10
26 begin
27 exec (@subquery);
28 set @execcount=@execcount+1
29 end