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Different strategies for removing duplicate records in SQL Server


By:   |   Read Comments (19)   |   Related Tips: More > Database Administration

Problem

In data warehousing applications during ETL (Extraction, Transformation and Loading) or even in OLTP (On Line Transaction Processing) applications we are often encountered with duplicate records in our table. To make the table data consistent and accurate we need to get rid of these duplicate records keeping only one of them in the table. In this tip I discuss different strategies which you can take for this, along with the pros and cons.

Solution

There are different methods for deleting duplicate (de-duplication) records from a table, each of them has its own pros and cons. I am going to discuss these methods, prerequisite of each of these methods along with its pros and cons.

  1. Using correlated subquery
  2. Using temporary table
  3. Creating new table with distinct records and renaming it..
  4. Using Common Table Expression (CTE)
  5. Using Fuzzy Group Transformation in SSIS
  6. Using MERGE Statement

1. Using correlated subquery

If you already have a identity column on your table, your work is half done. You can use a correlated subquery to get rid of the duplicates.

First let me briefly tell you how a correlated subquery works. In a correlated subquery, first outer query is evaluated, the result from the outer query is used by an inner sub query for its evaluation, whatever the outcome of the inner sub-query is again used by the outer query to get the final resultset. To learn more about correlated subqueries, you can click here.

In the example below, for the data deletion I am joining the inner query columns with the outer query to find the record with the maximum ID (you can even use minimum also and change the predicate to ">" from "<").  Then I am deleting all the records which has an ID less than what we have got from the inner query.

Please note, this approach can be taken only if you have identity column on the target table or you are willing to alter your target table to add an identity column which would require ALTER TABLE permission.

--Script #1 - De-duplication with correlated subquery

CREATE TABLE Employee
( 
[ID] INT IDENTITY, 
[FirstName] Varchar(100), 
[LastName] Varchar(100), 
[Address] Varchar(100), 
) 
GO 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Linda', 'Mitchel', 'America') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Linda', 'Mitchel', 'America') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
GO 
SELECT * FROM Employee 
GO 
--Selecting distinct records 
SELECT * FROM Employee E1 
WHERE E1.ID = ( SELECT MAX(ID) FROM Employee E2 
WHERE E2.FirstName = E1.FirstName AND E1.LastName = E2.LastName 
AND E1.Address = E2.Address) 
GO 
--Deleting duplicates 
DELETE Employee 
WHERE ID < ( SELECT MAX(ID) FROM Employee E2 
WHERE E2.FirstName = Employee.FirstName AND E2.LastName = Employee.LastName 
AND E2.Address = Employee.Address) 
GO 
SELECT * FROM Employee 
GO    

2. Using temporary table

In this approach we pull distinct records from the target table into a temporary table, then truncate the target table and finally insert the records from the temporary table back to the target table as you can see in Script #3.

Three things you need to be aware of when you are using this approach.

  • First you need to make sure you have or set enough size for tempdb database to hold all the distinct records especially if it is very large result-set.
  • Second you need to make sure you perform this operation in a transaction, at least the TRUNCATE and INSERT parts so that you are not left with an another problem if it fails in between for any reason.
  • Third you need to have the required permissions for object creation/truncation.

Script #2, creates a table and inserts some records along with some duplicate records which we will be using in all further examples.

--Script #2 - Creating a table with duplicate records

CREATE TABLE Employee
( 
[FirstName] Varchar(100), 
[LastName] Varchar(100), 
[Address] Varchar(100), 
) 
GO 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Linda', 'Mitchel', 'America') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Linda', 'Mitchel', 'America') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('John', 'Albert', 'Australia') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
INSERT INTO Employee([FirstName], [LastName], [Address]) 
VALUES ('Arshad', 'Ali', 'India') 
GO 
SELECT * FROM Employee 
GO 
--Script #3 - Using temporary table

BEGIN TRAN
-- Pull distinct records in the temporary table 
SELECT DISTINCT * INTO #Employee 
FROM Employee 
--Truncate the target table 
TRUNCATE TABLE Employee 
--Insert the distinct records from temporary table 
--back to target table 
INSERT INTO Employee SELECT * FROM #Employee 
--Drop the temporary table 
IF OBJECT_ID('tempdb..#Employee') IS NOT NULL 
  DROP TABLE #Employee 
COMMIT TRAN
GO 

SELECT * FROM Employee 
GO 

3. Creating new table with distinct records and renaming it

In this approach we create a new table with all distinct records, drop the existing target table and rename the newly created table with the original target table name. Please note, with this approach the meta-data about the target table will change for example object id, object creation date etc. so if you have any dependencies on these you have to take them into consideration.

Three things you need to aware of when you are using this approach.

  • First you need to make sure you have enough space in your database in the default filgroup (if you want your new table to be on some other file group than the default filegroup then you need to create a table first and then use INSERT INTO....SELECT * FROM) to hold all the distinct records especially if it is very large result-set.
  • Second you need to make sure you perform this operation in a transaction, at least the DROP and RENAME part so that you are not left with an another problem if it fails in between for any reason.
  • Third you need to have required permissions for object creation/drop.
--Script #4 - New table with distinct only

BEGIN TRAN
-- Pull distinct records in a new table 
SELECT DISTINCT * INTO EmployeeNew 
FROM Employee 
--Drop the old target table 
DROP TABLE Employee 
--rename the new table 
EXEC sp_rename 'EmployeeNew', 'Employee' 
COMMIT TRAN
GO 
SELECT * FROM Employee 
GO

4. Using Common Table Expression (CTE)

SQL Server 2005 introduced Common Table Expression (CTE) which acts as a temporary result set that is defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement.

In this example I am using a CTE for de-duplication. I am using the ROW_NUMBER function to return the sequential number of each row within a partition of a result set which is a grouping based on [FirstName], [LastName], [Address] columns (or columns of the table) and then I am deleting all records except where the sequential number is 1. This means keeping one record from the group and deleting all other similar/duplicate records. This is one of the efficient methods to delete records and I would suggest using this if you have SQL Server 2005 or 2008.

--Script #5 - Using CTE for de-duplication

--example 1
WITH CTE AS 
( 
SELECT ROW_NUMBER() OVER 
(PARTITION BY [FirstName], [LastName], [Address] 
Order BY [FirstName] DESC, [LastName] DESC, [Address] DESC ) 
AS RowNumber 
FROM Employee tbl 
WHERE EXISTS (SELECT TOP 1 1 FROM (SELECT FirstName,LastName,Address 
FROM Employee 
GROUP BY [FirstName], [LastName], [Address] HAVING COUNT(*) > 1 ) GrpTable 
WHERE GrpTable.FirstName = tbl.FirstName AND 
GrpTable.LastName = tbl.LastName AND GrpTable.Address = tbl.Address) 
) 
DELETE FROM CTE Where RowNumber > 1
GO 
SELECT * FROM Employee 
GO  
  
--A more simplified and faster example
WITH CTE AS 
( 
SELECT ROW_NUMBER() OVER 
(PARTITION BY [FirstName], [LastName], [Address] 
Order BY [FirstName] DESC, [LastName] DESC, [Address] DESC ) 
AS RowNumber, 
[FirstName], [LastName], [Address] 
FROM Employee tbl ) 
DELETE FROM CTE Where RowNumber > 1
GO 
SELECT * FROM Employee 
GO 

5. Using Fuzzy Group Transformation in SSIS

If you are using SSIS to upload data to your target table, you can use a Fuzzy Grouping Transformation before inserting records to the destination table to ignore duplicate records and insert only unique records. Here, in the image below, you can see 9 records are coming from source, but only 3 records are being inserted into the target table, that's because only 3 records are unique out of the 9 records. Refer to Script #2 above to see more about these 9 records that were used.

fuzzy grouping

In the Fuzzy Grouping Transformation editor, on the Columns tab you specify the columns which you want to be included in grouping.  As you can see in the below image I have chosen all 3 columns in my consideration for grouping.

fuzzy grouping transformation editor

In the Fuzzy Grouping Transformation, you might add a conditional split to direct unique rows or duplicate rows to two destinations. Here in the example you can see I am routing all the unique rows to the destination table and ignoring the duplicate records. The Fuzzy Grouping Transformation produces a few additional columns like _key_in which uniquely identifies each rows, _key_out which identifies a group of duplicate records etc.

conditional split transformation editor

6. Using MERGE Statement

Beginning with SQL Server 2008, now you can use MERGE SQL command to perform INSERT/UPDATE/DELETE operations in a single statement. This new command is similar to the UPSERT (fusion of the words UPDATE and INSERT.) command of Oracle.  It inserts rows that don't exist and updates the rows that do exist. With the introduction of the MERGE SQL command, developers can more effectively handle common data warehousing scenarios, like checking whether a row exists and then executing an insert or update or delete.

The MERGE statement basically merges data from a source result set to a target table based on a condition that you specify and if the data from the source already exists in the target or not. The new SQL command combines the sequence of conditional INSERT, UPDATE and DELETE commands in a single atomic statement, depending on the existence of a record. With this you can make sure no duplicate records are being inserted into the target table, but rather updated if there is any change and only new records are inserted which do not already exist in the target. For more information about this you can click here.

Next Steps


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About the author
MSSQLTips author Arshad Ali Arshad Ali is a SQL and BI Developer focusing on Data Warehousing projects for Microsoft.

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Monday, March 07, 2016 - 5:11:55 AM - Sam Back To Top

 
Hi,

I have a table which has columns like

 

firstname middle name  last name full name
ram lax  man ramlaxmanKumar
raghu null raghav raghuraghavRaja

 i need to remove the first name from full name if it is repeating in the full name. In teradata

 


Tuesday, December 01, 2015 - 3:35:04 PM - pradeep kumar Back To Top
Below is my table
 
Name1	Name2	Distance
BG	CH	280
CH 	BG	280
CH 	HY	350
HY	CH	350
CH 	CBE	500
 
I want remove duplicates in it How can i do it?
 
In above table describes distance between two cities, name1 is city1 & name2 is City2
 
Urgent help and attention is required using query..,
 
Thanks in Advance

Friday, November 27, 2015 - 10:15:45 AM - Kai Back To Top

Hello Arshad,

 

How do I remove dupes of this sort using standard SQL sans CTE?

Say, I have the follow data set.  Please reply directly to kai_tham@yahoo.com as well.

 

Thanks,

 

Kai

Clearly 

11000971 11000972 and 
11000972 11000971 are dupes.
11000971 11000972 1 1
11000972 11000971 1 1
11000973 11000974 25 25
11000974 11000973 25 25
11000981 11000982 1 1
11000982 11000981 1 1
11000989 11000990 1 1
11000989 11000992 1 1
11000990 11000989 1 1
11000990 11000991 1 1
11000991 11000990 1 1
11000991 11000992 1 1
11000992 11000989 1 1
11000992 11000991 1 1

Monday, March 02, 2015 - 10:44:08 AM - Pavan Back To Top

Hi Arshad ,

I work in a DW environment where in i get a requirment of deleting duplicate records within a huge set of data, which of the above method would be the fastest?

Regards,

Pavan

 

 

 


Thursday, September 25, 2014 - 2:35:18 PM - Crayon Back To Top

 

Hi Arshad,

First of all, thanks for your great post.

I believe, the problem in the 'Temporary table' example, there's a problem when Orginal table get rows from Temp Table.

It give me this Error:

Msg 8101, Level 16, State 1, Line 1
An explicit value for the identity column in table 'Employee_dupli' can only be specified when a column list is used and IDENTITY_INSERT is ON.

Please figure out what next must be done. Thank you.


Monday, September 15, 2014 - 10:16:03 PM - Enrique Back To Top

Thanks, this is the most useful and clear example In how to fix a problem and how to explain it in the best way possible, your post is elegant and do the job nicely.

I'm very thankful with you,

Best regards,

 

 


Wednesday, June 05, 2013 - 11:30:17 PM - VALABOJU NARESH Back To Top

Thanks for providing multiple solutions


Monday, April 22, 2013 - 10:21:12 AM - Wayne Back To Top

Thank you very much. Your Script #5 CTE code worked a charm. I could't get several other sites' suggested code to work for my purposes.


Monday, March 04, 2013 - 9:12:12 AM - shikha tiwari Back To Top

How can I delete duplicates query using  from UNION ALL function?


Monday, February 18, 2013 - 3:53:42 AM - kapil Gautam Back To Top

thanks for the easy and best solution for deletion of duplicate row ,Is the same method is also applicable for the coloums also or need to apply othe one


Saturday, December 22, 2012 - 1:35:59 PM - pradeep kumar N Back To Top

thanks for the simple solution for deleting the duplicate rows from a table


Thursday, September 06, 2012 - 4:33:29 PM - Nagarjuna Back To Top

Hello Arshad ...

 

It is very good script to remove the dulipicate values ...

 

Geret Job..

 

Thanks!

 

 

 

 


Friday, February 17, 2012 - 5:05:43 AM - Bruce Back To Top
Great Script Example!!! I used the correlated subquery method to remove +/- 50000 duplicates, worked like a charm! Thanks!

Friday, December 16, 2011 - 11:39:33 PM - Arshad Back To Top

Yes you are right we need to use DISTINCT to remove duplicates and get only one record and this is what I am doing in the script.

 

Please note, the table which I have used in the query does not have ID column which has different value for the same employee as indicated in your query. If this is the case, first I wouldn't say that is duplicate in in all cases  but if stil it is, then yes you have to use MIN or MAX with GROUP BY as you have indicated in the above script or some technic like using correlated query etc.

Hope it clears your doubt. 


Friday, December 16, 2011 - 11:24:54 PM - Arshad Back To Top

Yes you are right we need to use DISTINCT to remove duplicates and get only one record and this is what I am doing in the script.

Please note, the table which I have used in the query does not have ID column which has different value for the same employee as indicated in your query. If this is the case, first I wouldn't say that is duplicate but if stil it is, then yes you have to use MIN or MAX with GROUP BY as you have indicated in the above script.

Hope it clears your doubt.


Friday, December 16, 2011 - 1:55:20 PM - Samuel Back To Top

 

In Using temporary table method, you have to capture the distinct records instead of retrieving all the records from the table.

i.e..,  Intead of "SELECT DISTINCT * INTO #Employee from Employee" you could have written

"SELECT MIN(E1.ID), E1.FirstName, E1.LastName, E1.Address FROM Employee E1 inner join Employee E2 

ON E2.FirstName = E1.FirstName AND E1.LastName = E2.LastName AND E1.Address = E2.Address

GROUP By E1.FirstName, E1.LastName, E1.Address"

 

And its the same case with the next method:    "Creating new table with distinct records and renaming it.."

 

Please let me know if you have questions...

 

Thanks,

Samuel

 


Friday, December 16, 2011 - 11:43:33 AM - Arshad Back To Top

Hi Samuel,

Thanks for your feedback!

I just checked the script you mentioned, it worked fine. Request you to, please, post the error/exception that you are getting so that I can tell you exactly what's going wrong.

 

 


Friday, December 16, 2011 - 10:50:50 AM - Samuel Back To Top

 

Something is also wrong in your "Creating new table with distinct records and renaming it.." script.. Please rewrite this, if you get a chance.


Friday, December 16, 2011 - 10:35:50 AM - Samuel Back To Top

 

Something is wrong in your temporary table script. If you get a chance, please make sure to correct it.

 

Thanks,

Suman


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