在SQL中删除重复记录

以表employee为例

SQL> desc employee
 

 Name                                          Type
 ----------------------------------------- -------- ------------------ 

emp_id                                                NUMBER(10)
emp_name                                              VARCHAR2(20)
salary                                                NUMBER(10,2) 

可以通过下面的语句查询重复的记录：
 

SQL> select * from employee;

    EMP_ID EMP_NAME                                  SALARY

---------- ---------------------------------------- ----------
 

         1 sunshine                                      10000
         1 sunshine                                      10000
         2 semon                                         20000
         2 semon                                         20000
         3 xyz                                           30000
         2 semon                                         20000 

SQL> select distinct * from employee;
 

    EMP_ID EMP_NAME                                     SALARY
---------- ---------------------------------------- ----------
         1 sunshine                                      10000
         2 semon                                         20000
         3 xyz                                           30000
 

SQL> select * from employee group by emp_id,emp_name,salary having count (*)>1
 

    EMP_ID EMP_NAME                                     SALARY
---------- ---------------------------------------- ----------
         1 sunshine                                      10000
         2 semon                                         20000

SQL> select * from employee e1
where rowid in (select max(rowid) from employe e2
 where e1.emp_id=e2.emp_id and
 e1.emp_name=e2.emp_name and e1.salary=e2.salary);

    EMP_ID EMP_NAME                                     SALARY
---------- ---------------------------------------- ----------
         1 sunshine                                      10000
         3 xyz                                           30000
         2 semon                                         20000
 

2. 删除的几种方法：

 （1）通过建立临时表来实现 

SQL>create table temp_emp as (select distinct * from employee) 

SQL> truncate table employee; (清空employee表的数据） 

SQL> insert into employee select * from temp_emp;  (再将临时表里的内容插回来）
 

( 2）通过唯一rowid实现删除重复记录.在Oracle中，每一条记录都有一个rowid，rowid在整个数据库中是唯一的，rowid确定了每条记录是在Oracle中的哪一个数据文件、块、行上。在重复的记录中，可能所有列的内容都相同，但rowid不会相同，所以只要确定出重复记录中那些具有最大或最小rowid的就可以了，其余全部删除。

SQL>delete from employee e2 where rowid not in (
        select max(e1.rowid) from employee e1 where
        e1.emp_id=e2.emp_id and e1.emp_name=e2.emp_name and e1.salary=e2.salary);--这里用min(rowid)也可以。

 

SQL>delete from employee e2 where rowid <(
        select max(e1.rowid) from employee e1 where
        e1.emp_id=e2.emp_id and e1.emp_name=e2.emp_name and e1.salary=e2.salary);

（3）也是通过rowid，但效率更高。 

SQL>delete from employee where rowid not in (
        select max(t1.rowid) from employee t1 group by 1.emp_id,t1.emp_name,t1.salary);
         --这里用min(rowid)也可以。

    EMP_ID EMP_NAME                                     SALARY

---------- ---------------------------------------- ----------
         1 sunshine                                      10000
         3 xyz                                           30000
         2 semon                                         20000