要讨论的问题:一对多关系,联接查询,只从子表当中挑选一行第一个表:CREATE TABLE [dbo].[t1](
[id] [int] IDENTITY(1,1) NOT NULL,
[name] [nvarchar](50) NULL,
)
第2个表:
CREATE TABLE [dbo].[t2](
[id] [int] IDENTITY(1,1) NOT NULL,
[t1_ID] [int] NULL,
[bwin] [int] NULL,
[dat] [datetime] NULL CONSTRAINT [DF_t2_dat] DEFAULT (getdate()),
)-- 成功的算法,
select t1.*,t2.id,t2.bwin,t2.dat
from t1 left join
(select * from t2 where [dat] IN (select Max(dat) from t2 group by t1_id) ) AS t2
on t1.id = t2.t1_id还有一种办法,通过Select计算获得t2的唯一值,这种算法只能获得一列
select t1.*,
(select TOP 1 t2.id from t2 where t2.t1_id= t1.id order by dat desc ) AS t2_id
from t1 2年以前,弄过一个差不多的东西,那个时候是我跟公司的几个同事分头写的,每个人都写出一种办法,但是时间久了,算法都给忘记了,那些人也联系不上了。如今到这里来试试,请过路的高手和神仙指教指教。
[id] [int] IDENTITY(1,1) NOT NULL,
[name] [nvarchar](50) NULL,
)
第2个表:
CREATE TABLE [dbo].[t2](
[id] [int] IDENTITY(1,1) NOT NULL,
[t1_ID] [int] NULL,
[bwin] [int] NULL,
[dat] [datetime] NULL CONSTRAINT [DF_t2_dat] DEFAULT (getdate()),
)-- 成功的算法,
select t1.*,t2.id,t2.bwin,t2.dat
from t1 left join
(select * from t2 where [dat] IN (select Max(dat) from t2 group by t1_id) ) AS t2
on t1.id = t2.t1_id还有一种办法,通过Select计算获得t2的唯一值,这种算法只能获得一列
select t1.*,
(select TOP 1 t2.id from t2 where t2.t1_id= t1.id order by dat desc ) AS t2_id
from t1 2年以前,弄过一个差不多的东西,那个时候是我跟公司的几个同事分头写的,每个人都写出一种办法,但是时间久了,算法都给忘记了,那些人也联系不上了。如今到这里来试试,请过路的高手和神仙指教指教。
select distinct t1.*,t2.bwin,t2.dat
from t1,t2
where t1.id = t2.t1_id
select distinct t1.*,t1.bwin,t2.dat,max(t2.id)ID
from t1,t2
where t1.id = t2.t1_id
group by t1.id,t1.name,t1.bwin,t1.dat
select distinct t1.*,t1.bwin,t2.dat,min(t2.id)ID
from t1,t2
where t1.id = t2.t1_id
group by t1.id,t1.name,t1.bwin,t1.dat
select * from t2 t where not exists(select 1 from t2 where t1_ID=t.t1_id and id<t.id)
)b on a.id=b.t1_id
from t1 outer apply
(select top(1)* from t2 where t1.id = t2.t1_id order by dat desc) AS t2
from t1 m, t2 n
where m.id = n.id and n.dat = (select max(dat) from t2 where id = n.id)select m.* , n.id,n.bwin,n.dat
from t1 m, t2 n
where m.id = n.id and not exists (select 1 from t2 where id = n.id and dat > n.dat)更多方法参考如下:--按某一字段分组取最大(小)值所在行的数据
--(爱新觉罗.毓华(十八年风雨,守得冰山雪莲花开) 2007-10-23于浙江杭州)
/*
数据如下:
name val memo
a 2 a2(a的第二个值)
a 1 a1--a的第一个值
a 3 a3:a的第三个值
b 1 b1--b的第一个值
b 3 b3:b的第三个值
b 2 b2b2b2b2
b 4 b4b4
b 5 b5b5b5b5b5
*/
--创建表并插入数据:
create table tb(name varchar(10),val int,memo varchar(20))
insert into tb values('a', 2, 'a2(a的第二个值)')
insert into tb values('a', 1, 'a1--a的第一个值')
insert into tb values('a', 3, 'a3:a的第三个值')
insert into tb values('b', 1, 'b1--b的第一个值')
insert into tb values('b', 3, 'b3:b的第三个值')
insert into tb values('b', 2, 'b2b2b2b2')
insert into tb values('b', 4, 'b4b4')
insert into tb values('b', 5, 'b5b5b5b5b5')
go--一、按name分组取val最大的值所在行的数据。
--方法1:
select a.* from tb a where val = (select max(val) from tb where name = a.name) order by a.name
--方法2:
select a.* from tb a where not exists(select 1 from tb where name = a.name and val > a.val)
--方法3:
select a.* from tb a,(select name,max(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name
--方法4:
select a.* from tb a inner join (select name , max(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name
--方法5
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name
/*
name val memo
---------- ----------- --------------------
a 3 a3:a的第三个值
b 5 b5b5b5b5b5
*/--二、按name分组取val最小的值所在行的数据。
--方法1:
select a.* from tb a where val = (select min(val) from tb where name = a.name) order by a.name
--方法2:
select a.* from tb a where not exists(select 1 from tb where name = a.name and val < a.val)
--方法3:
select a.* from tb a,(select name,min(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name
--方法4:
select a.* from tb a inner join (select name , min(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name
--方法5
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val < a.val) order by a.name
/*
name val memo
---------- ----------- --------------------
a 1 a1--a的第一个值
b 1 b1--b的第一个值
*/--三、按name分组取第一次出现的行所在的数据。
select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name
/*
name val memo
---------- ----------- --------------------
a 2 a2(a的第二个值)
b 1 b1--b的第一个值
*/--四、按name分组随机取一条数据。
select a.* from tb a where val = (select top 1 val from tb where name = a.name order by newid()) order by a.name
/*
name val memo
---------- ----------- --------------------
a 1 a1--a的第一个值
b 5 b5b5b5b5b5
*/--五、按name分组取最小的两个(N个)val
select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val < a.val ) order by a.name,a.val
select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val) order by a.name,a.val
select a.* from tb a where exists (select count(*) from tb where name = a.name and val < a.val having Count(*) < 2) order by a.name,a.val
/*
name val memo
---------- ----------- --------------------
a 1 a1--a的第一个值
a 2 a2(a的第二个值)
b 1 b1--b的第一个值
b 2 b2b2b2b2
*/--六、按name分组取最大的两个(N个)val
select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name,a.val
select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val desc) order by a.name,a.val
select a.* from tb a where exists (select count(*) from tb where name = a.name and val > a.val having Count(*) < 2) order by a.name , a.val
/*
name val memo
---------- ----------- --------------------
a 2 a2(a的第二个值)
a 3 a3:a的第三个值
b 4 b4b4
b 5 b5b5b5b5b5
*/
--七,如果整行数据有重复,所有的列都相同。
/*
数据如下:
name val memo
a 2 a2(a的第二个值)
a 1 a1--a的第一个值
a 1 a1--a的第一个值
a 3 a3:a的第三个值
a 3 a3:a的第三个值
b 1 b1--b的第一个值
b 3 b3:b的第三个值
b 2 b2b2b2b2
b 4 b4b4
b 5 b5b5b5b5b5
*/
--在sql server 2000中只能用一个临时表来解决,生成一个自增列,先对val取最大或最小,然后再通过自增列来取数据。
--创建表并插入数据:
create table tb(name varchar(10),val int,memo varchar(20))
insert into tb values('a', 2, 'a2(a的第二个值)')
insert into tb values('a', 1, 'a1--a的第一个值')
insert into tb values('a', 1, 'a1--a的第一个值')
insert into tb values('a', 3, 'a3:a的第三个值')
insert into tb values('a', 3, 'a3:a的第三个值')
insert into tb values('b', 1, 'b1--b的第一个值')
insert into tb values('b', 3, 'b3:b的第三个值')
insert into tb values('b', 2, 'b2b2b2b2')
insert into tb values('b', 4, 'b4b4')
insert into tb values('b', 5, 'b5b5b5b5b5')
goselect * , px = identity(int,1,1) into tmp from tbselect m.name,m.val,m.memo from
(
select t.* from tmp t where val = (select min(val) from tmp where name = t.name)
) m where px = (select min(px) from
(
select t.* from tmp t where val = (select min(val) from tmp where name = t.name)
) n where n.name = m.name)drop table tb,tmp/*
name val memo
---------- ----------- --------------------
a 1 a1--a的第一个值
b 1 b1--b的第一个值(2 行受影响)
*/
--在sql server 2005中可以使用row_number函数,不需要使用临时表。
--创建表并插入数据:
create table tb(name varchar(10),val int,memo varchar(20))
insert into tb values('a', 2, 'a2(a的第二个值)')
insert into tb values('a', 1, 'a1--a的第一个值')
insert into tb values('a', 1, 'a1--a的第一个值')
insert into tb values('a', 3, 'a3:a的第三个值')
insert into tb values('a', 3, 'a3:a的第三个值')
insert into tb values('b', 1, 'b1--b的第一个值')
insert into tb values('b', 3, 'b3:b的第三个值')
insert into tb values('b', 2, 'b2b2b2b2')
insert into tb values('b', 4, 'b4b4')
insert into tb values('b', 5, 'b5b5b5b5b5')
goselect m.name,m.val,m.memo from
(
select * , px = row_number() over(order by name , val) from tb
) m where px = (select min(px) from
(
select * , px = row_number() over(order by name , val) from tb
) n where n.name = m.name)drop table tb/*
name val memo
---------- ----------- --------------------
a 1 a1--a的第一个值
b 1 b1--b的第一个值(2 行受影响)
*/
1楼、2楼、3楼都是AcHerat的回复,你真是个热心的人,不过很遗憾地告诉你,都不行。
1楼获得的结果是子表(t2)中的所有行,而不是我说的“挑选一行”。
2、3楼则不能正常运行,因为bwin是t2表中的,t1没有。正确的结果是:4楼的2个算法和7楼的结果也是这样的。5楼的结果:
9楼的结果:
这一贴来简单回复一下:
1楼、2楼、3楼都是AcHerat的回复,你真是个热心的人,不过很遗憾地告诉你,都不行。
1楼获得的结果是子表(t2)中的所有行,而不是我说的“挑选一行”。
2、3楼则不能正常运行,因为bwin是t2表中的,t1没有。正确的结果是:4楼的2个算法和7楼的结果也是这样的。5楼的结果:
9楼的结果:
from t1,t2
where t1.id = t2.t1_id
正确结果你可以看一下16楼.
BTW:4楼是版主哦,难怪这么厉害!如果版主回来的话,能不能顺便把10楼、11楼给删除了?