SELECT *
FROM tb_music_mp3
WHERE (songtype IN
(SELECT songtype
FROM tb_music_mp3
GROUP BY songtype
HAVING (COUNT(songtype) > 1)))

SELECT *
FROM tb_usertable_other_100000
WHERE (userid IN
(SELECT userid
FROM tb_usertable_other_100000
GROUP BY userid
HAVING (COUNT(userid) > 1)))

select *
FROM tb_usertable_main
WHERE (userid IN
(SELECT userid
FROM tb_usertable_main
GROUP BY userid
HAVING (COUNT(userid) > 1)))
order by userid

delete
FROM tb_usertable_other_5600000
WHERE (id NOT IN
(SELECT MIN(id)
FROM tb_usertable_other_5600000
GROUP BY userid))

在大的數據庫應用中，經常因為各種原因遇到重復的記錄，造成數據的冗余和維護上的不便。
id custom
1 化工公司
2 化工公司
3 化工公司
4 軟件公司
5 制藥公司
6 制藥公司

現在想將重復的留下一條，重復的刪除，結果如下：
id custom
1 化工公司
4 軟件公司
5 制藥公司

delete 表 where id not in(select min(id) from 表 group by custom)

SELECT *
FROM tb_music_mp3

SELECT id, songtype, songname
FROM tb_music_mp3 a
WHERE (NOT EXISTS
(SELECT 1
FROM tb_music_mp3 b
WHERE a.songtype = b.songtype AND a.id > b.id))

1.用rowid方法

2.用group by方法

3.用distinct方法

1。用rowid方法

據據oracle帶的rowid屬性，進行判斷，是否存在重復,語句如下：
查數據:
select * from table1 a where rowid !=(select max(rowid)
from table1 b where a.name1=b.name1 and a.name2=b.name2......)
刪數據：
delete from table1 a where rowid !=(select max(rowid)
from table1 b where a.name1=b.name1 and a.name2=b.name2......)

2.group by方法

查數據:
　　select count(num), max(name) from student --列出重復的記錄數，並列出他的name屬性
　　group by num
　　having count(num) >1 --按num分組後找出表中num列重復，即出現次數大於一次
刪數據：
　　delete c
　　這樣的話就把所有重復的都刪除了。

3.用distinct方法 -對於小的表比較有用

create table table_new as select distinct * from table1 minux
truncate table table1;
insert into table1 select * from table_new;

delete
FROM tb_book_list
WHERE (id NOT IN
(SELECT max(id)
FROM tb_book_list
GROUP BY bookid))
(5065 行受影響)

sql 消除重復