Remove Duplicate Rows from a Table in SQL Server

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?

Solution

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

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

Now the table data will be like the following table:

EMPLOYEE_ID	ATTENDANCE_DATE	AUTOID
A001	2011-01-01	1
A001	2011-01-01	2
A002	2011-01-01	3
A002	2011-01-01	4
A002	2011-01-01	5
A003	2011-01-01	6

Check the AUTOID column. Now we will start playing the game with this column.

Now use the following code to find out the duplicate rows that exist in the table.

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

The above code will give us the following result:

EMPLOYEE_ID	ATTENDANCE_DATE	AUTOID
A001	2011-01-01	2
A002	2011-01-01	4
A002	2011-01-01	5

Ultimately, these are the duplicate rows which we want to delete to resolve the issue. Use the following code to resolve it.

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

Now check the data. No duplicate rows exist in the table.