详见博客地址:http://blog.csdn.net/feixianxxx/archive/2010/03/21/5402391.aspx
- SQL code
/*----------------------------------*auther:Poofly*date:2010.3.14*VERSION: Microsoft SQL Server 2008 (RTM) - 10.0.1600.22 (Intel X86) Jul 19 2008 Copyright (c) 1988-2008 Microsoft Corporation Enterprise Evaluation Edition on Windows NT 6.1 <X86> (Build 7600: )*转载请注明出处*更多精彩内容,请进http://blog.csdn.net/feixianxxx------------------------------------*/--收集一些东西来写写
--1.EXCEPT
注意点:2个NULL值对于EXCEPT是相等的,不同于一般的筛选器.
- SQL code
--环境create table test_1 (a int ,b int)create table test_2 (c int, d int)insert test_1 select 1,2 union all select 1,null union all select 3,4insert test_2 select 1,2 union all select 1,null-- except select * from test_1 except select * from test_2/*a b----------- -----------3 4*/-- not existsselect * from test_1where not exists(select * from test_2 where a=c and b=d)/*a b----------- -----------1 NULL --这条记录对于test_1来说是唯一的3 4*/
ps:因为现有版本不支持 except all 所以EXCEPT使用时候会有一个排序的阶段,效率一般不太好
--2.OVER()子句
注意点:OVER()子句在多聚合情况下比在select 下的子查询效率 or GROUP BY 高很多
- SQL code
--环境create table test_3( id int, value int)insert test_3 values(1,12)insert test_3 values(1,1)insert test_3 values(1,3)insert test_3 values(1,2)insert test_3 values(1,6)insert test_3 values(2,1)insert test_3 values(2,2)insert test_3 values(2,4)insert test_3 values(2,3)go--OVERSELECT ID,[SUM]=SUM(VALUE) OVER(),[AVG]=AVG(VALUE) OVER(),[COUNT]=COUNT(VALUE) OVER(),[MAX]=MAX(VALUE) OVER()FROM test_3 --子查询select id,[SUM]=(select SUM(VALUE) from test_3 where l.id=id),[AVG]=(select AVG(VALUE) O from test_3 where l.id=id),[COUNT]=(select COUNT(VALUE) from test_3 where l.id=id),[MAX]=(select MAX(VALUE) from test_3 where l.id=id)FROM test_3 l--group by SELECT ID,[SUM]=SUM(VALUE),[AVG]=AVG(VALUE) ,[COUNT]=COUNT(VALUE) ,[MAX]=MAX(VALUE) FROM test_3 group by id
CTRL+L 可以发现 over的开销明显小于后者..其中子查询效率极差
--3.利用字符串解决带附加属性的问题
注意点:当需要附加属性来解决问题时候,一般会想到子查询,但是因为子查询需要良好的索引设计,所以不太好用.可以使用字符串来解决这个问题
- SQL code
--环境create table test_4 (id int, a int, b int , c int)insert test_4 select 1,2,3,4 union all select 1,3,5,4 union all select 1,3,7,4 union all select 1,3,7,8 union all select 2,2,3,4 union all select 2,5,3,8 union all select 2,5,3,8 union all select 2,7,3,8 union all select 2,1,9,9 go--字符串select ID,a=SUBSTRING(COL,1,5),b=SUBSTRING(COL,6,5),c=SUBSTRING(COL,11,5)from (select ID,MAX(CAST(a as char(5))+CAST(b as char(5))+CAST(c as char(5))) as colfrom test_4 group by ID) l--子查询select * from test_4 kwhere not exists(select * from test_4 where id=k.id and (a>k.a or a=k.a and b>k.b or a=k.a and b=k.b and c>k.c))/*ID a b c----------- ---------- ---------- ----------1 3 7 8 2 7 3 8 */
--4.利用聚合实现字符串拼接
注意:不用XML、函数、临时表、游标去实现字符串的拼接
- SQL code
--环境create table test_5(empid int, name varchar(10))insert test_5 select 1,'a'union all select 1,'b' union all select 1,'c'union all select 1,'d'union all select 2,'a' union all select 2,'t'union all select 2,'v' select empid,name=MAX(case when rn=1 then name else '' end)+MAX(case when rn=2 then ','+name else '' end)+MAX(case when rn=3 then ','+name else '' end)+MAX(case when rn=4 then ','+name else '' end) from(select empid,name,(select COUNT(*) from test_5 where k.empid=empid and k.name>=name) as rn from test_5 k )z group by empid--xmlselect empid,name=stuff((select ','+name as [text()] from test_5 where k.empid=empid order by name for XML PATH('')),1,1,'')from test_5 kgroup by empid/*empid name----------- -------------------------------------------1 a,b,c,d2 a,t,v*/