表DUPES有7条记录，现在删除NAME字段有重复的，只保留一条。
删除之后，我ROLLBACK，再查，表怎么空了？
SQL> select * from dupes;
ID NAME
---------- ----------
1 NAPOLEON
2 DYNAMITE
3 DYNAMITE
4 SHE SELLS
5 SEA SHELLS
6 SEA SHELLS
7 SEA SHELLS
7 rows selected.
SQL> delete from dupes where id not in ( select min(id) from dupes group by name ) ;
3 rows deleted.
SQL> select * from dupes;
ID NAME
---------- ----------
1 NAPOLEON
2 DYNAMITE
4 SHE SELLS
5 SEA SHELLS
SQL> rollback;
Rollback complete.
SQL> select * from dupes;
no rows selected
