求一SQL语句，请人帮忙，删掉重复记录
表中数据如下：
id     username     course         result
1           jacky       English       80
2           jacky       Math             85
3           tom           English       99
4           tom           Math             86
5           jacky       English       80
6           Tina         English       85
我想得到
id     username     course         result
1           jacky       English       80
2           jacky       Math             85
3           tom           English       99
4           tom           Math             86
6           Tina         English       85
就是把username     course   result   全部一样的数据只保留一条。如何做？谢谢各位了。

------解决方案--------------------
Delete From 表 Where id Not In (Select Min(id) From 表 Group By username, course, result)
------解决方案--------------------
一张表里面以两个字段为唯一字段，当几条记录的这两个字段完全相同时，需要删除重复项，如下表
a b c d
1 2 3 4
1 5 3 5
1 2 7 9
以a、b为唯一字段，第一条和第三条的a、b完全相同，所以，需要删除第一条记录1 2 3 4 或者第三条记录1 2 7 9
即如下结果：
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

请问各位大侠这种sql语句怎么写


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

如果要同时删除第一和第三行
即如下结果：
a b c d
1 5 3 5

语句如下：

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


------------------------------------------------
在几千条记录里,存在着些相同的记录,如何能用SQL语句,删除掉重复的呢?谢谢!
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)

2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录
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、查找表中多余的重复记录