Remove Duplicate Rows from a Table in SQL Server

Firstly, we will create a table, where we will insert some duplicate rows to understand the topic properly. Create a table called ATTENDANCE by using the following code:

CREATE TABLE [dbo].[ATTENDANCE](
[EMPLOYEE_ID] [varchar](50) NOT NULL,
[ATTENDANCE_DATE] [date] NOT NULL
) ON [PRIMARY]

Now insert some data into this table.

INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A001',CONVERT(DATETIME,'01-01-11',5))
INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A001',CONVERT(DATETIME,'01-01-11',5))
INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A002',CONVERT(DATETIME,'01-01-11',5))
INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A002',CONVERT(DATETIME,'01-01-11',5))
INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A002',CONVERT(DATETIME,'01-01-11',5))
INSERT INTO dbo.ATTENDANCE (EMPLOYEE_ID,ATTENDANCE_DATE)VALUES
('A003',CONVERT(DATETIME,'01-01-11',5))

After inserting the data, check the data of the below table. If we grouped the employee_id and attendance_date, then A001 and A002 become duplicates.
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:

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.

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.

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.

Is it too complicated? I dont think so 🙂