If you are dealing with duplicate rows in a SQL server database, and you’re not sure how to find them, don’t panic. I had to deal with a similar situation recently.
Two weeks ago, I was working on a project where I had to import some data from a legacy system into a whole new table. After the data had been loaded and a report was generated with that data, we’ve noticed there were some duplicate entries. Therefore, I had to quickly figure out how to put together a quick query to find duplicate rows and removed them from the table.

In this post I will show simple steps to take in case you are facing the same issue. In order to get this post, I am going to create a new table and populate it with some seed data.

Create Database

DROP TABLE IF EXISTS Students;
CREATE TABLE Students (
    Id INT IDENTITY(1, 1), 
    First_Name  nvarchar(100), 
    Last_Name  nvarchar(100), 
    Phone nvarchar(12),
    PRIMARY KEY(id)
);

Populate the table with data

Now that we have our table added, I will proceed to populate it with some data for our demonstration.

   INSERT INTO dbo.Students
        (first_name,last_name,phone) 
   VALUES
        ('Mary','Allan','407-455-1414'),
	('John','Doe','407-445-5414'),
        ('Chirac','Paul','407-455-1814'),
	('John','Doe','407-445-5414'),
        ('Mela','Jil','407-787-6767'),
	('John','Doe','407-445-5414'),
        ('Mary','Allan','407-455-1414'),
	('John','Doe','407-445-5414'),
        ('Mary','Allan','407-455-1414'),
	('Chris','Tai','407-555-7777'),
	('Albert','Allan','321-455-9977'),
	('Jean','Doe','407-445-5414');

Ensure the data was inserted properly

Okay, we have the data we need in the table, let’s run a quick query to see if it match our criteria, in order words have some duplicates. Let’s run a simple select statement to retrieve the full data-set in the the table.

   SELECT Id, First_Name, Last_Name,Phone 
   FROM dbo.students
   ORDER BY First_Name, Last_Name desc;

Find duplicate rows in the table

As a matter of fact, there are many ways to find and return the duplicates. I ended up using group by to get my result, If you end up using some other method, please share it in the comment below.

 SELECT  First_Name, 
          Last_Name,
          Phone,  
          COUNT(*) Occurrences
  FROM dbo.students
  GROUP BY
    First_Name, 
    Last_Name,
    Phone
  HAVING 
    COUNT(*) > 1;

Last, below you can see the result of the duplicated rows found and the number of time.