Monday, 24 December 2012

Delete duplicate rows with no primary key on a SQL Server table


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 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 
DELETE FROM dbo.duplicateTest WHERE ID 
SET ROWCOUNT 

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(1FROM dbo.duplicateTest WHERE ID 
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.


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Hi i am Muthu kumar,software engineer working on PL/SQL,ASP.Net,VB.Net,C#.Net,SQL Server.