Removing the duplicate rows from a database

[Bangaru]

If you have a problem in deleting duplicate rows in a table if the table does not have any primary key, then here is a tip to do that.

Deleting duplicate rows when there is no primary key

Problem

Every once in awhile a table gets created without a primary key and duplicate records get entered. The problem gets even worse when you have two identical rows in the table and there is no way to distinguish between the two rows. So how do you delete the duplicate record?

Solution

One option that SQL Server gives you is the ability to set ROWCOUNT which limits the numbers of records affected by a command. The default value is 0 which means all records, but this value can be set prior to running a command. So let's create a table and add 4 records with one duplicate record.

Create a table called duplicateTest and add 4 records.

CREATE TABLE dbo.duplicateTest 
( 
   [ID] [int] , 
   [FirstName] [varchar](25), 
   [LastName] [varchar](25) 
) ON [PRIMARY] 
 
INSERT INTO dbo.duplicateTest VALUES(1, 'Bob','Smith') 
 
INSERT INTO dbo.duplicateTest VALUES(2, 'Dave','Jones') 
 
INSERT INTO dbo.duplicateTest VALUES(3, 'Karen','White') 
 
INSERT INTO dbo.duplicateTest VALUES(1, 'Bob','Smith') 
 

If we select all data we get the following:

 
SELECT * FROM dbo.duplicateTest 

ID FirstName  LastName 
 
1  Bob       Smith 
 
2  Dave      Jones 
 
3  Karen     White 
 
1  Bob       Smith
 

If we try to select the record for Bob Smith will all of the available values such as the following query:

 
SELECT * FROM dbo.duplicateTest WHERE ID = 1 AND FirstName = 'Bob' AND LastName = 'Smith' 

We still get 2 rows of data:

ID FirstName LastName 
 
1  Bob       Smith 
 
1  Bob       Smith
 

So to delete the duplicate record with SQL Server 2000 and 2005 we can use the SET ROWCOUNT command to limit the number of rows affected by a query. By setting it to 1 we can just delete one of these rows in the table. Note: the select commands are just used to show the data prior and after the delete occurs.

 
SELECT * FROM dbo.duplicateTest 
 
SET ROWCOUNT 1 
 
DELETE FROM dbo.duplicateTest WHERE ID = 1 
 
SET ROWCOUNT 0 
 
SELECT * FROM dbo.duplicateTest 
 

With SQL Server 2005 we can also use the TOP command when we issue the delete, such as the following. Note: the select commands are just used to show the data prior and after the delete occurs.

SELECT * FROM dbo.duplicateTest 
 
DELETE TOP(1) FROM dbo.duplicateTest WHERE ID = 1 
 
SELECT * FROM dbo.duplicateTest 
 

So as you can see with SQL Server 2005 there are two options to allow you to delete duplicate identical rows of data in your tables.

Here is one note from Microsoft about using SET ROWCOUNT:

Using SET ROWCOUNT will not affect DELETE, INSERT, and UPDATE statements in the next release of SQL Server. Avoid using SET ROWCOUNT together with DELETE, INSERT, and UPDATE statements in new development work, and plan to modify applications that currently use it. Also, for DELETE, INSERT, and UPDATE statements that currently use SET ROWCOUNT, we recommend that you rewrite them to use the TOP syntax.

Next Steps

Take a look how the ROWCOUNT command can be used to affect the results of your query

Also take a look at the TOP command and changes that have been implemented with SQL Server 2005

Start using TOP instead of ROWCOUNT for SQL Server 2005 and later

Hope this will be useful.

Best Regards,
Bangaar

[mabubakarpk]

Very Welldon nice learning material.

[mathewgk80]

Hi,

I think the same material is available in

http://www.mssqltips.com/tip.asp?tip=1103

you can read tht also....

Regards,
Mathew