# sql server – How can I remove duplicate rows?

## The Question :

1303 people think this question is useful

What is the best way to remove duplicate rows from a fairly large SQL Server table (i.e. 300,000+ rows)?

The rows, of course, will not be perfect duplicates because of the existence of the RowID identity field.

MyTable

RowID int not null identity(1,1) primary key,
Col1 varchar(20) not null,
Col2 varchar(2048) not null,
Col3 tinyint not null


• Quick tip for PostgreSQL users reading this (lots, going by how often it’s linked to): Pg doesn’t expose CTE terms as updatable views so you can’t DELETE FROM a CTE term directly. See stackoverflow.com/q/18439054/398670
• @CraigRinger the same is true for Sybase – I have collected the remaining solutions here (should be valid for PG and others, too: stackoverflow.com/q/19544489/1855801 (just replace the ROWID() function by the RowID column, if any)
• Just to add a caveat here. When running any de-duplication process, always double check what you are deleting first! This is one of those areas where it is very common to accidentally delete good data.

1152 people think this answer is useful

Assuming no nulls, you GROUP BY the unique columns, and SELECT the MIN (or MAX) RowId as the row to keep. Then, just delete everything that didn’t have a row id:

DELETE FROM MyTable
LEFT OUTER JOIN (
SELECT MIN(RowId) as RowId, Col1, Col2, Col3
FROM MyTable
GROUP BY Col1, Col2, Col3
) as KeepRows ON
MyTable.RowId = KeepRows.RowId
WHERE
KeepRows.RowId IS NULL



In case you have a GUID instead of an integer, you can replace

MIN(RowId)



with

CONVERT(uniqueidentifier, MIN(CONVERT(char(36), MyGuidColumn)))



767 people think this answer is useful

Another possible way of doing this is

;

--Ensure that any immediately preceding statement is terminated with a semicolon above
WITH cte
AS (SELECT ROW_NUMBER() OVER (PARTITION BY Col1, Col2, Col3
ORDER BY ( SELECT 0)) RN
FROM   #MyTable)
DELETE FROM cte
WHERE  RN > 1;



I am using ORDER BY (SELECT 0) above as it is arbitrary which row to preserve in the event of a tie.

To preserve the latest one in RowID order for example you could use ORDER BY RowID DESC

Execution Plans

The execution plan for this is often simpler and more efficient than that in the accepted answer as it does not require the self join.

This is not always the case however. One place where the GROUP BY solution might be preferred is situations where a hash aggregate would be chosen in preference to a stream aggregate.

The ROW_NUMBER solution will always give pretty much the same plan whereas the GROUP BY strategy is more flexible.

Factors which might favour the hash aggregate approach would be

• No useful index on the partitioning columns
• relatively fewer groups with relatively more duplicates in each group

In extreme versions of this second case (if there are very few groups with many duplicates in each) one could also consider simply inserting the rows to keep into a new table then TRUNCATE-ing the original and copying them back to minimise logging compared to deleting a very high proportion of the rows.

153 people think this answer is useful

There’s a good article on removing duplicates on the Microsoft Support site. It’s pretty conservative – they have you do everything in separate steps – but it should work well against large tables.

I’ve used self-joins to do this in the past, although it could probably be prettied up with a HAVING clause:

DELETE dupes
FROM MyTable dupes, MyTable fullTable
WHERE dupes.dupField = fullTable.dupField
AND dupes.secondDupField = fullTable.secondDupField
AND dupes.uniqueField > fullTable.uniqueField



98 people think this answer is useful

The following query is useful to delete duplicate rows. The table in this example has ID as an identity column and the columns which have duplicate data are Column1, Column2 and Column3.

DELETE FROM TableName
WHERE  ID NOT IN (SELECT MAX(ID)
FROM   TableName
GROUP  BY Column1,
Column2,
Column3
/*Even if ID is not null-able SQL Server treats MAX(ID) as potentially
nullable. Because of semantics of NOT IN (NULL) including the clause
below can simplify the plan*/
HAVING MAX(ID) IS NOT NULL)



The following script shows usage of GROUP BY, HAVING, ORDER BY in one query, and returns the results with duplicate column and its count.

SELECT YourColumnName,
COUNT(*) TotalCount
FROM   YourTableName
GROUP  BY YourColumnName
HAVING COUNT(*) > 1
ORDER  BY COUNT(*) DESC



63 people think this answer is useful
delete t1
from table t1, table t2
where t1.columnA = t2.columnA
and t1.rowid>t2.rowid



Postgres:

delete
from table t1
using table t2
where t1.columnA = t2.columnA
and t1.rowid > t2.rowid



44 people think this answer is useful
DELETE LU
FROM   (SELECT *,
Row_number()
OVER (
partition BY col1, col1, col3
ORDER BY rowid DESC) [Row]
FROM   mytable) LU
WHERE  [row] > 1



40 people think this answer is useful

This will delete duplicate rows, except the first row

DELETE
FROM
Mytable
WHERE
RowID NOT IN (
SELECT
MIN(RowID)
FROM
Mytable
GROUP BY
Col1,
Col2,
Col3
)



35 people think this answer is useful

I would prefer CTE for deleting duplicate rows from sql server table

by keeping original

WITH CTE AS
(
SELECT *,ROW_NUMBER() OVER (PARTITION BY col1,col2,col3 ORDER BY col1,col2,col3) AS RN
FROM MyTable
)

DELETE FROM CTE WHERE RN<>1



without keeping original

WITH CTE AS
(SELECT *,R=RANK() OVER (ORDER BY col1,col2,col3)
FROM MyTable)

DELETE CTE
WHERE R IN (SELECT R FROM CTE GROUP BY R HAVING COUNT(*)>1)



28 people think this answer is useful

To Fetch Duplicate Rows:

SELECT
name, email, COUNT(*)
FROM
users
GROUP BY
name, email
HAVING COUNT(*) > 1



To Delete the Duplicate Rows:

DELETE users
WHERE rowid NOT IN
(SELECT MIN(rowid)
FROM users
GROUP BY name, email);



23 people think this answer is useful

Quick and Dirty to delete exact duplicated rows (for small tables):

select  distinct * into t2 from t1;
delete from t1;
insert into t1 select *  from t2;
drop table t2;



21 people think this answer is useful

I prefer the subquery\having count(*) > 1 solution to the inner join because I found it easier to read and it was very easy to turn into a SELECT statement to verify what would be deleted before you run it.

--DELETE FROM table1
--WHERE id IN (
SELECT MIN(id) FROM table1
GROUP BY col1, col2, col3
-- could add a WHERE clause here to further filter
HAVING count(*) > 1
--)



17 people think this answer is useful
SELECT  DISTINCT *
INTO tempdb.dbo.tmpTable
FROM myTable

TRUNCATE TABLE myTable
INSERT INTO myTable SELECT * FROM tempdb.dbo.tmpTable
DROP TABLE tempdb.dbo.tmpTable



15 people think this answer is useful

I thought I’d share my solution since it works under special circumstances. I my case the table with duplicate values did not have a foreign key (because the values were duplicated from another db).

begin transaction
-- create temp table with identical structure as source table
Select * Into #temp From tableName Where 1 = 2

-- insert distinct values into temp
insert into #temp
select distinct *
from  tableName

-- delete from source
delete from tableName

-- insert into source from temp
insert into tableName
select *
from #temp

rollback transaction
-- if this works, change rollback to commit and execute again to keep you changes!!



PS: when working on things like this I always use a transaction, this not only ensures everything is executed as a whole, but also allows me to test without risking anything. But off course you should take a backup anyway just to be sure…

14 people think this answer is useful

This query showed very good performance for me:

DELETE tbl
FROM
MyTable tbl
WHERE
EXISTS (
SELECT
*
FROM
MyTable tbl2
WHERE
tbl2.SameValue = tbl.SameValue
AND tbl.IdUniqueValue < tbl2.IdUniqueValue
)



it deleted 1M rows in little more than 30sec from a table of 2M (50% duplicates)

14 people think this answer is useful

Using CTE. The idea is to join on one or more columns that form a duplicate record and then remove whichever you like:

;with cte as (
select
min(PrimaryKey) as PrimaryKey
UniqueColumn1,
UniqueColumn2
from dbo.DuplicatesTable
group by
UniqueColumn1, UniqueColumn1
having count(*) > 1
)
delete d
from dbo.DuplicatesTable d
inner join cte on
d.PrimaryKey > cte.PrimaryKey and
d.UniqueColumn1 = cte.UniqueColumn1 and
d.UniqueColumn2 = cte.UniqueColumn2;



13 people think this answer is useful

Yet another easy solution can be found at the link pasted here. This one easy to grasp and seems to be effective for most of the similar problems. It is for SQL Server though but the concept used is more than acceptable.

Here are the relevant portions from the linked page:

Consider this data:

EMPLOYEE_ID ATTENDANCE_DATE
A001    2011-01-01
A001    2011-01-01
A002    2011-01-01
A002    2011-01-01
A002    2011-01-01
A003    2011-01-01



So how can we delete those duplicate data?

First, insert an identity column in that table by using the following code:

ALTER TABLE dbo.ATTENDANCE ADD AUTOID INT IDENTITY(1,1)



Use the following code to resolve it:

DELETE FROM dbo.ATTENDANCE WHERE AUTOID NOT IN (SELECT MIN(AUTOID) _
FROM dbo.ATTENDANCE GROUP BY EMPLOYEE_ID,ATTENDANCE_DATE)



12 people think this answer is useful

Here is another good article on removing duplicates.

It discusses why its hard: “SQL is based on relational algebra, and duplicates cannot occur in relational algebra, because duplicates are not allowed in a set.

The temp table solution, and two mysql examples.

In the future are you going to prevent it at a database level, or from an application perspective. I would suggest the database level because your database should be responsible for maintaining referential integrity, developers just will cause problems 😉

12 people think this answer is useful

Oh sure. Use a temp table. If you want a single, not-very-performant statement that “works” you can go with:

DELETE FROM MyTable WHERE NOT RowID IN
(SELECT
(SELECT TOP 1 RowID FROM MyTable mt2
WHERE mt2.Col1 = mt.Col1
AND mt2.Col2 = mt.Col2
AND mt2.Col3 = mt.Col3)
FROM MyTable mt)



Basically, for each row in the table, the sub-select finds the top RowID of all rows that are exactly like the row under consideration. So you end up with a list of RowIDs that represent the “original” non-duplicated rows.

11 people think this answer is useful

I had a table where I needed to preserve non-duplicate rows. I’m not sure on the speed or efficiency.

DELETE FROM myTable WHERE RowID IN (
SELECT MIN(RowID) AS IDNo FROM myTable
GROUP BY Col1, Col2, Col3
HAVING COUNT(*) = 2 )



11 people think this answer is useful

Use this

WITH tblTemp as
(
SELECT ROW_NUMBER() Over(PARTITION BY Name,Department ORDER BY Name)
As RowNumber,* FROM <table_name>
)
DELETE FROM tblTemp where RowNumber >1



10 people think this answer is useful

The other way is Create a new table with same fields and with Unique Index. Then move all data from old table to new table. Automatically SQL SERVER ignore (there is also an option about what to do if there will be a duplicate value: ignore, interrupt or sth) duplicate values. So we have the same table without duplicate rows. If you don’t want Unique Index, after the transfer data you can drop it.

Especially for larger tables you may use DTS (SSIS package to import/export data) in order to transfer all data rapidly to your new uniquely indexed table. For 7 million row it takes just a few minute.

10 people think this answer is useful

This is the easiest way to delete duplicate record

 DELETE FROM tblemp WHERE id IN
(
SELECT MIN(id) FROM tblemp
GROUP BY  title HAVING COUNT(id)>1
)



9 people think this answer is useful

By useing below query we can able to delete duplicate records based on the single column or multiple column. below query is deleting based on two columns. table name is: testing and column names empno,empname

DELETE FROM testing WHERE empno not IN (SELECT empno FROM (SELECT empno, ROW_NUMBER() OVER (PARTITION BY empno ORDER BY empno)
AS [ItemNumber] FROM testing) a WHERE ItemNumber > 1)
or empname not in
(select empname from (select empname,row_number() over(PARTITION BY empno ORDER BY empno)
AS [ItemNumber] FROM testing) a WHERE ItemNumber > 1)



9 people think this answer is useful
1. Create new blank table with the same structure

2. Execute query like this

INSERT INTO tc_category1
SELECT *
FROM tc_category
GROUP BY category_id, application_id
HAVING count(*) > 1


3. Then execute this query

INSERT INTO tc_category1
SELECT *
FROM tc_category
GROUP BY category_id, application_id
HAVING count(*) = 1



7 people think this answer is useful

I would mention this approach as well as it can be helpful, and works in all SQL servers: Pretty often there is only one – two duplicates, and Ids and count of duplicates are known. In this case:

SET ROWCOUNT 1 -- or set to number of rows to be deleted
delete from myTable where RowId = DuplicatedID
SET ROWCOUNT 0



7 people think this answer is useful

From the application level (unfortunately). I agree that the proper way to prevent duplication is at the database level through the use of a unique index, but in SQL Server 2005, an index is allowed to be only 900 bytes, and my varchar(2048) field blows that away.

I dunno how well it would perform, but I think you could write a trigger to enforce this, even if you couldn’t do it directly with an index. Something like:

-- given a table stories(story_id int not null primary key, story varchar(max) not null)
CREATE TRIGGER prevent_plagiarism
ON stories
after INSERT, UPDATE
AS
DECLARE @cnt AS INT

SELECT @cnt = Count(*)
FROM   stories
INNER JOIN inserted
ON ( stories.story = inserted.story
AND stories.story_id != inserted.story_id )

IF @cnt > 0
BEGIN
RAISERROR('plagiarism detected',16,1)

ROLLBACK TRANSACTION
END



Also, varchar(2048) sounds fishy to me (some things in life are 2048 bytes, but it’s pretty uncommon); should it really not be varchar(max)?

7 people think this answer is useful

Another way of doing this :–

DELETE A
FROM   TABLE A,
TABLE B
WHERE  A.COL1 = B.COL1
AND A.COL2 = B.COL2
AND A.UNIQUEFIELD > B.UNIQUEFIELD



7 people think this answer is useful
DELETE
FROM
table_name T1
WHERE
rowid > (
SELECT
min(rowid)
FROM
table_name T2
WHERE
T1.column_name = T2.column_name
);



6 people think this answer is useful
CREATE TABLE car(Id int identity(1,1), PersonId int, CarId int)

INSERT INTO car(PersonId,CarId)
VALUES(1,2),(1,3),(1,2),(2,4)

--SELECT * FROM car

;WITH CTE as(
SELECT ROW_NUMBER() over (PARTITION BY personid,carid order by personid,carid) as rn,Id,PersonID,CarId from car)

DELETE FROM car where Id in(SELECT Id FROM CTE WHERE rn>1)



6 people think this answer is useful

I you want to preview the rows you are about to remove and keep control over which of the duplicate rows to keep. See http://developer.azurewebsites.net/2014/09/better-sql-group-by-find-duplicate-data/

with MYCTE as (
SELECT ROW_NUMBER() OVER (
PARTITION BY DuplicateKey1
,DuplicateKey2 -- optional
ORDER BY CreatedAt -- the first row among duplicates will be kept, other rows will be removed
) RN
FROM MyTable
)
DELETE FROM MYCTE
WHERE RN > 1