2015.8.31
查询目标
DeleteDuplicate Emails
Writea SQL query to delete all duplicate email entries in a table named Person,keeping only unique emails based on its smallest Id.
题目来自 Leetcode
Person表如下：
SQL> select * fromperson;
ID EMAIL
------------------------------------------------------------
1 [email protected]
2 [email protected]
3 [email protected]
建表及插入测试数据
droptable person purge;
createtable person(id int,email char(50));
insertinto person(id,email) values(1,‘[email protected]‘);
insertinto person(id,email) values(2,‘[email protected]‘);
insertinto person(id,email) values(3,‘[email protected]‘);
得到的Person表中共3行数据。根据题目要求，第三行数据应该被删除。
SQL语句如下（oracle中执行）：
方法一：使用MIN
SQL>delete from person a where a.id > (
select min(b.id) from person b where a.email=b.email
);
方法二：或者用 ANY
SQL>delete from person a where a.id > any (
select b.id from person bwhere a.email=b.email
);
方法三：使用GROUP BY & ROWID
SQL>deletefrom person where rowid not in(select min(rowid) from person group by email);
方法四：self-join
SQL>delete from person p1 where rowid not in (select min(rowid)from person p2 where p1.email=p2.email);
方法五：使用 ROW_NUMBER()
SQL > delete from person where rowid in (select rid from(select rowid rid , row_number() over(partition by email order by email)row_num from person) where row_num>1);
方法六：使用DENSE_RANK()
delete from person where rowid in (select rid from (select rowidrid , dense_rank() over(partition by email order by rowid) rank fromperson) where rank>1);
参考文章：
http://www.dba-oracle.com/t_delete_duplicate_table_rows.htm
http://sqlandplsql.com/2013/01/29/5-ways-to-delete-duplicate-records-oracle/
主要内容如下：
In Oracle there are many ways to delete duplicate records. Notethat below example are described to just explain the different possibilities.
Considerthe EMP table with below rows
createtable emp(
EMPNNO integer,
EMPNAME varchar2(20),
SALARY number);
10 Bill 2000
11 Bill 2000
12 Mark 3000
12 Mark 3000
12 Mark 3000
13 Tom 4000
14 Tom 5000
15 Susan 5000
1. Usingrowid
SQL >delete from emp
where rowid not in
(select max(rowid) from emp group by empno);
Thistechnique can be applied to almost scenarios. Group by operation should be onthe columns which identify the duplicates.
2. Usingself-join
SQL> delete from emp e1
where rowid not in
(select max(rowid) from emp e2
where e1.empno = e2.empno );
3. Using row_number()
SQL> delete from emp where rowid in
(
select rid from
(
select rowid rid,
row_number() over(partition by empno order by empno) rn
from emp
)
where rn > 1
);
This isanother efficient way to delete duplicates
4. Usingdense_rank()
SQL> delete from emp where rowid in
(
select rid from
(
select rowid rid,
dense_rank() over(partition by empno order by rowid) rn
from emp
)
where rn > 1
);
Here youcan use both rank() and dens_rank() since both will give unique records whenorder by rowid.
5. Using groupby
Considerthe EMP table with below rows
10 Bill 2000
11 Bill 2000
12 Mark 3000
13 Mark 3000
SQL> delete from emp where
(empno,empname,salary) in
(
select max(empno),empname,salary from emp
group by empname,salary
);
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