去掉重复记录
有两个意义上的重复记录，一是完全重复的记录，也即所有字段均重复的记录，二是部分关键字段重复的记录，比如Name字段重复，而其他字段不一定重复或都重复可以忽略。
1、对于第一种重复，比较容易解决，使用
select distinct * from tableName
就可以得到无重复记录的结果集。
如果该表需要删除重复的记录(重复记录保留1条)，可以按以下方法删除
select distinct * into #Tmp from tableName
truncate table tableName
select * into tableName from #Tmp
drop table #Tmp
发生这种重复的原因是表设计不周产生的，增加唯一索引列即可解决。
2、这类重复问题通常要求保留重复记录中的第一条记录，操作方法如下
假设有重复的字段为Name,Address，要求得到这两个字段唯一的结果集
select identity(int,1,1) as autoID, * into #Tmp from tableName
select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,autoID
select * from #Tmp where autoID in(select autoID from #tmp2)
最后一个select即得到了Name，Address不重复的结果集(但多了一个autoID字段，实际写时可以写在select子句中省去此列)
--按某一字段分组取最大(小)值所在行的数据(爱新觉罗.毓华 2007-10-23于浙江杭州)/*数据如下：name val memoa 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 b2b2b2b2b 4 b4b4b 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5select 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5select 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)valselect a.* from tb a where 2 > (select count(*) from tb where name = a.name and val < a.val ) order by a.name,a.valselect 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.valselect 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/*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)valselect a.* from tb a where 2 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name,a.valselect 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.valselect 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/*name val memo ---------- ----------- -------------------- a 2 a2(a的第二个值)a 3 a3:a的第三个值b 4 b4b4b 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 memoa 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 b2b2b2b2b 4 b4b4b 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)*/--按某一字段分组取最大(小)值所在行的数据(2007-10-23于浙江杭州)/*数据如下：name val memoa 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 b2b2b2b2b 4 b4b4b 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5select 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5select 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)valselect a.* from tb a where 2 > (select count(*) from tb where name = a.name and val < a.val ) order by a.name,a.valselect 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.valselect 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/*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)valselect a.* from tb a where 2 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name,a.valselect 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.valselect 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/*name val memo ---------- ----------- -------------------- a 2 a2(a的第二个值)a 3 a3:a的第三个值b 4 b4b4b 5 b5b5b5b5b5*/declare @tb table(JobID int,jobCorporation int,JobCount int,JobStart datetime,JobBack int)insert into @tb select 1,11,0,'2004-4-7',1 insert into @tb select 2,11,0,'2005-4-4',1 insert into @tb select 3,22,0,'2005-4-4',1 insert into @tb select 4,22,0,'2005-4-4',1 insert into @tb select 5,22,0,'2005-4-4',1select * from @tb t where not exists(select 1 from @tb where jobCorporation=t.jobCorporation and JobID<t.JobID)