學(xué)習(xí)sql有一段時(shí)間了，發(fā)現(xiàn)在我建了一個(gè)用來(lái)測(cè)試的表（沒(méi)有建索引）中出現(xiàn)了許多的重復(fù)記錄。后來(lái)總結(jié)了一些刪除重復(fù)記錄的方法，在Oracle中，可以通過(guò)唯一rowid實(shí)現(xiàn)刪除重復(fù)記錄；還可以建臨時(shí)表來(lái)實(shí)現(xiàn)...這個(gè)只提到其中的幾種簡(jiǎn)單實(shí)用的方法，希望可以和大家分享（以表employee為例）。

SQL> desc employee

?Name????????????????????????????????????? Null???? Type
?----------------------------------------- -------- ------------------

emp_id??????????????????????????????????????????????? NUMBER(10)
emp_name?????????????????????????????????????????? VARCHAR2(20)

salary???????????????????????????????????????????????? ?NUMBER(10,2)

?

?

可以通過(guò)下面的語(yǔ)句查詢(xún)重復(fù)的記錄：

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. 刪除的幾種方法：

?

（1）通過(guò)建立臨時(shí)表來(lái)實(shí)現(xiàn)

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

SQL> truncate table employee; (清空employee表的數(shù)據(jù)）

SQL> insert into employee select * from temp_emp; ?(再將臨時(shí)表里的內(nèi)容插回來(lái)）

?

( 2）通過(guò)唯一rowid實(shí)現(xiàn)刪除重復(fù)記錄.在Oracle中，每一條記錄都有一個(gè)rowid，rowid在整個(gè)數(shù)據(jù)庫(kù)中是唯一的，rowid確定了每條記錄是在Oracle中的哪一個(gè)數(shù)據(jù)文件、塊、行上。在重復(fù)的記錄中，可能所有列的內(nèi)容都相同，但rowid不會(huì)相同，所以只要確定出重復(fù)記錄中那些具有最大或最小rowid的就可以了，其余全部刪除。

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);--這里用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）也是通過(guò)rowid，但效率更高。

SQL>delete from employee where rowid not in (
??????? select max(t1.rowid) from employee t1 group by

???????? t1.emp_id,t1.emp_name,t1.salary);--這里用min(rowid)也可以。

?

?

??? EMP_ID EMP_NAME???????????????????????????????????? SALARY

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

???????? 1 sunshine????????????????????????????????????? 10000

???????? 3 xyz???? ??????????????????????????????????????? 30000

???????? 2 semon???????????????????????????????????????? 20000

?

?

?

?

SQL> desc employee

?Name????????????????????????????????????? Null???? Type
?----------------------------------------- -------- ------------------

emp_id??????????????????????????????????????????????? NUMBER(10)
emp_name?????????????????????????????????????????? VARCHAR2(20)

salary???????????????????????????????????????????????? ?NUMBER(10,2)

?

?

可以通過(guò)下面的語(yǔ)句查詢(xún)重復(fù)的記錄：

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. 刪除的幾種方法：

?

（1）通過(guò)建立臨時(shí)表來(lái)實(shí)現(xiàn)

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

SQL> truncate table employee; (清空employee表的數(shù)據(jù)）

SQL> insert into employee select * from temp_emp; ?(再將臨時(shí)表里的內(nèi)容插回來(lái)）

?

( 2）通過(guò)唯一rowid實(shí)現(xiàn)刪除重復(fù)記錄.在Oracle中，每一條記錄都有一個(gè)rowid，rowid在整個(gè)數(shù)據(jù)庫(kù)中是唯一的，rowid確定了每條記錄是在Oracle中的哪一個(gè)數(shù)據(jù)文件、塊、行上。在重復(fù)的記錄中，可能所有列的內(nèi)容都相同，但rowid不會(huì)相同，所以只要確定出重復(fù)記錄中那些具有最大或最小rowid的就可以了，其余全部刪除。

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);--這里用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）也是通過(guò)rowid，但效率更高。

SQL>delete from employee where rowid not in (
??????? select max(t1.rowid) from employee t1 group by

???????? t1.emp_id,t1.emp_name,t1.salary);--這里用min(rowid)也可以。

?

?

??? EMP_ID EMP_NAME???????????????????????????????????? SALARY

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

???????? 1 sunshine????????????????????????????????????? 10000

???????? 3 xyz???? ??????????????????????????????????????? 30000

???????? 2 semon???????????????????????????????????????? 20000

?

?

?

?

?

?

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