Removing duplicate rows from table in Oracle - sql

Question

I'm testing something in Oracle and populated a table with some sample data, but in the process I accidentally loaded duplicate records, so now I can't create a primary key using some of the columns.
How can I delete all duplicate rows and leave only one of them?

Answer 1

Use the rowid pseudocolumn.
DELETE FROM your_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM your_table
GROUP BY column1, column2, column3);
Where column1, column2, and column3 make up the identifying key for each record. You might list all your columns.

Answer 2

From Ask Tom
delete from t
where rowid IN ( select rid
from (select rowid rid,
row_number() over (partition by
companyid, agentid, class , status, terminationdate
order by rowid) rn
from t)
where rn <> 1);
(fixed the missing parenthesis)

Answer 3

From DevX.com:
DELETE FROM our_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM our_table
GROUP BY column1, column2, column3...) ;
Where column1, column2, etc. is the key you want to use.

Answer 4

DELETE FROM tablename a
WHERE a.ROWID > ANY (SELECT b.ROWID
FROM tablename b
WHERE a.fieldname = b.fieldname
AND a.fieldname2 = b.fieldname2)

Answer 5

Solution 1)
delete from emp
where rowid not in
(select max(rowid) from emp group by empno);
Solution 2)
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
);
Solution 3)
delete from emp e1
where rowid not in
(select max(rowid) from emp e2
where e1.empno = e2.empno );

Answer 6

create table t2 as select distinct * from t1;

Answer 7

You should do a small pl/sql block using a cursor for loop and delete the rows you don't want to keep. For instance:
declare
prev_var my_table.var1%TYPE;
begin
for t in (select var1 from my_table order by var 1) LOOP
-- if previous var equal current var, delete the row, else keep on going.
end loop;
end;

Answer 8

To select the duplicates only the query format can be:
SELECT GroupFunction(column1), GroupFunction(column2),...,
COUNT(column1), column1, column2...
FROM our_table
GROUP BY column1, column2, column3...
HAVING COUNT(column1) > 1
So the correct query as per other suggestion is:
DELETE FROM tablename a
WHERE a.ROWID > ANY (SELECT b.ROWID
FROM tablename b
WHERE a.fieldname = b.fieldname
AND a.fieldname2 = b.fieldname2
AND ....so on.. to identify the duplicate rows....)
This query will keep the oldest record in the database for the criteria chosen in the WHERE CLAUSE.
Oracle Certified Associate (2008)

Answer 9

create table abcd(id number(10),name varchar2(20))
insert into abcd values(1,'abc')
insert into abcd values(2,'pqr')
insert into abcd values(3,'xyz')
insert into abcd values(1,'abc')
insert into abcd values(2,'pqr')
insert into abcd values(3,'xyz')
select * from abcd
id Name
1 abc
2 pqr
3 xyz
1 abc
2 pqr
3 xyz
Delete Duplicate record but keep Distinct Record in table
DELETE
FROM abcd a
WHERE ROWID > (SELECT MIN(ROWID) FROM abcd b
WHERE b.id=a.id
);
run the above query 3 rows delete
select * from abcd
id Name
1 abc
2 pqr
3 xyz

Answer 10

This blog post was really helpful for general cases:
If the rows are fully duplicated (all values in all columns can have copies) there are no columns to use! But to keep one you still need a unique identifier for each row in each group.
Fortunately, Oracle already has something you can use. The rowid.
All rows in Oracle have a rowid. This is a physical locator. That is, it states where on disk Oracle stores the row. This unique to each row. So you can use this value to identify and remove copies. To do this, replace min() with min(rowid) in the uncorrelated delete:
delete films
where rowid not in (
select min(rowid)
from films
group by title, uk_release_date
)

Answer 11

The Fastest way for really big tables
Create exception table with structure below:
exceptions_table
ROW_ID ROWID
OWNER VARCHAR2(30)
TABLE_NAME VARCHAR2(30)
CONSTRAINT VARCHAR2(30)
Try create a unique constraint or primary key which will be violated by the duplicates. You will get an error message because you have duplicates. The exceptions table will contain
the rowids for the duplicate rows.
alter table add constraint
unique --or primary key
(dupfield1,dupfield2) exceptions into exceptions_table;
Join your table with exceptions_table by rowid and delete dups
delete original_dups where rowid in (select ROW_ID from exceptions_table);
If the amount of rows to delete is big, then create a new table (with all grants and indexes) anti-joining with exceptions_table by rowid and rename the original table into original_dups table and rename new_table_with_no_dups into original table
create table new_table_with_no_dups AS (
select field1, field2 ........
from original_dups t1
where not exists ( select null from exceptions_table T2 where t1.rowid = t2.row_id )
)

Answer 12

Using rowid-
delete from emp
where rowid not in
(select max(rowid) from emp group by empno);
Using self join-
delete from emp e1
where rowid not in
(select max(rowid) from emp e2
where e1.empno = e2.empno );

Answer 13

Solution 4)
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
);

Answer 14

1. solution
delete from emp
where rowid not in
(select max(rowid) from emp group by empno);
2. sloution
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
);
3.solution
delete from emp e1
where rowid not in
(select max(rowid) from emp e2
where e1.empno = e2.empno );
4. solution
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
);

Answer 15

5. solution
delete from emp where rowid in
(
select rid from
(
select rowid rid,rank() over (partition by emp_id order by rowid)rn from emp
)
where rn > 1
);

Answer 16

DELETE from table_name where rowid not in (select min(rowid) FROM table_name group by column_name);
and you can also delete duplicate records in another way
DELETE from table_name a where rowid > (select min(rowid) FROM table_name b where a.column=b.column);

Answer 17

DELETE FROM tableName WHERE ROWID NOT IN (SELECT MIN (ROWID) FROM table GROUP BY columnname);

Answer 18

delete from dept
where rowid in (
select rowid
from dept
minus
select max(rowid)
from dept
group by DEPTNO, DNAME, LOC
);

Answer 19

For best performance, here is what I wrote :
(see execution plan)
DELETE FROM your_table
WHERE rowid IN
(select t1.rowid from your_table t1
LEFT OUTER JOIN (
SELECT MIN(rowid) as rowid, column1,column2, column3
FROM your_table
GROUP BY column1, column2, column3
) co1 ON (t1.rowid = co1.rowid)
WHERE co1.rowid IS NULL
);

Answer 20

Check below scripts -
1.
Create table test(id int,sal int);
2.
insert into test values(1,100);
insert into test values(1,100);
insert into test values(2,200);
insert into test values(2,200);
insert into test values(3,300);
insert into test values(3,300);
commit;
3.
select * from test;
You will see here 6-records.
4.run below query -
delete from
test
where rowid in
(select rowid from
(select
rowid,
row_number()
over
(partition by id order by sal) dup
from test)
where dup > 1)
select * from test;
You will see that duplicate records have been deleted.
Hope this solves your query.
Thanks :)

Answer 21

I didn't see any answers that use common table expressions and window functions.
This is what I find easiest to work with.
DELETE FROM
YourTable
WHERE
ROWID IN
(WITH Duplicates
AS (SELECT
ROWID RID,
ROW_NUMBER()
OVER(
PARTITION BY First_Name, Last_Name, Birth_Date)
AS RN
SUM(1)
OVER(
PARTITION BY First_Name, Last_Name, Birth_Date
ORDER BY ROWID ROWS BETWEEN UNBOUNDED PRECEDING
AND UNBOUNDED FOLLOWING)
AS CNT
FROM
YourTable
WHERE
Load_Date IS NULL)
SELECT
RID
FROM
duplicates
WHERE
RN > 1);
Somethings to note:
1) We are only checking for duplication on the fields in the partition clause.
2) If you have some reason to pick one duplicate over others you can use an order by clause to make that row will have row_number() = 1
3) You can change the number duplicate preserved by changing the final where clause to "Where RN > N" with N >= 1 (I was thinking N = 0 would delete all rows that have duplicates, but it would just delete all rows).
4) Added the Sum partition field the CTE query which will tag each row with the number rows in the group. So to select rows with duplicates, including the first item use "WHERE cnt > 1".

Answer 22

solution :
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
);

Answer 23

create or replace procedure delete_duplicate_enq as
cursor c1 is
select *
from enquiry;
begin
for z in c1 loop
delete enquiry
where enquiry.enquiryno = z.enquiryno
and rowid > any
(select rowid
from enquiry
where enquiry.enquiryno = z.enquiryno);
end loop;
end delete_duplicate_enq;

Answer 24

This is similar to the top answer but gives me a much better explain plan:
delete from your_table
where rowid in (
select max(rowid)
from your_table
group by column1, column2, column3
having count(*) > 1
);