如何在同一個數據庫表中查詢出相同的記錄

詳細說明如下:
--如何按字段刪除重復記錄

一張表里面以兩個字段為唯一字段，當幾條記錄的這兩個字段完全相同時，需要刪除重復項，如下表
a b c d
1 2 3 4
1 5 3 5
1 2 7 9
以a、b為唯一字段，第一條和第三條的a、b完全相同，所以，需要刪除第一條記錄1 2 3 4 或者第三條記錄1 2 7 9
即如下結果：
a b c d
1 2 3 4
1 5 3 5
或
a b c d
1 5 3 5
1 2 7 9

請問各位大俠這種sql語句怎么寫


CREATE TABLE Tb1(id int, [a] varchar(255), [b] varchar(255), [c] varchar(255), [d] varchar(255))
INSERT Tb1(id, [a], [b], [c], [d])
  SELECT 1, '1','2','3','4'
UNION ALL SELECT 2, '1','5','3','5'
UNION ALL SELECT 3, '1','2','7','9'
UNION ALL SELECT 4, '1','4','7','6'

delete Tb1 where [id] not in (select max([id]) from Tb1 group by a,b )
select * from tb1
 
drop table tb1

如果要同時刪除第一和第三行
即如下結果：
a b c d
1 5 3 5

語句如下：

delete m from tb t
inner join
(
select a ,b
from tb
group by a , b
having count(*)>1
)n
on m.a = n.a and m.b = n.b 
或
delete * from tb as m,
(
select a ,b
from tb
group by a , b
having count(*)>1
)n
where m.a = n.a and m.b = n.b 








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在幾千條記錄里,存在着些相同的記錄,如何能用SQL語句,刪除掉重復的呢?謝謝!
1、查找表中多余的重復記錄，重復記錄是根據單個字段（peopleId）來判斷
select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)

2、刪除表中多余的重復記錄，重復記錄是根據單個字段（peopleId）來判斷，只留有rowid最小的記錄
delete from people 
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1)

3、查找表中多余的重復記錄（多個字段） 
select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)

4、刪除表中多余的重復記錄（多個字段），只留有rowid最小的記錄
delete from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

5、查找表中多余的重復記錄（多個字段），不包含rowid最小的記錄
select * from vitae a
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

比方說在A表中存在一個字段“name”，而且不同記錄之間的“name”值有可能會相同，
現在就是需要查詢出在該表中的各記錄之間，“name”值存在重復的項；
Select Name,Count(*) From A Group By Name Having Count(*) > 1

如果還查性別也相同大則如下:
Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1
------------------------------------------------------------------------------------------------
declare @max integer,@id integer
declare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段 having count(*) >； 1
open cur_rows
fetch cur_rows into @id,@max
while @@fetch_status=0
begin
select @max = @max -1
set rowcount @max
delete from 表名 where 主字段 = @id
fetch cur_rows into @id,@max
end
close cur_rows
set rowcount 0

方法二
　　有兩個意義上的重復記錄，一是完全重復的記錄，也即所有字段均重復的記錄，二是部分關鍵字段重復的記錄，比如Name字段重復，而其他字段不一定重復或都重復可以忽略。
　　1、對於第一種重復，比較容易解決，使用
select distinct * from tableName
　　就可以得到無重復記錄的結果集。
　　如果該表需要刪除重復的記錄（重復記錄保留1條），可以按以下方法刪除
select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp
　　發生這種重復的原因是表設計不周產生的，增加唯一索引列即可解決。

　　2、這類重復問題通常要求保留重復記錄中的第一條記錄，操作方法如下
　　假設有重復的字段為Name,Address，要求得到這兩個字段唯一的結果集
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)
　　最后一個select即得到了Name，Address不重復的結果集（但多了一個autoID字段，實際寫時可以寫在select子句中省去此列）
select * from tablename where id in (
select id from tablename 
group by id 
having count(id) > 1)