How to Find Duplicate Values in SQL Server

In SQL Server, it is quite common to have duplicate values in a table. These duplicates can cause issues in data analysis, reporting, and other operations. Therefore, it is important to identify and remove them. In this blog post, we will discuss how to find duplicate values in SQL Server.

There are different ways to identify duplicate values in SQL Server, but one of the most commonly used methods is to use the GROUP BY clause and the HAVING clause.

Let’s say we have a table called “customers” with the following columns:

• id (int)
• name (varchar)
• email (varchar)
• phone (varchar)

To find the duplicate email addresses in the customers table, we can use the following SQL query:

SELECT email, COUNT(*) as cnt
FROM customers
GROUP BY email
HAVING COUNT(*) > 1;

This query groups the rows by the email column and counts the number of times each email appears in the table. The HAVING clause filters the results to only show email addresses that appear more than once.

If we want to see all the duplicate rows (not just the email column), we can modify the query as follows:

SELECT *
FROM customers
WHERE email IN (
  SELECT email
  FROM customers
  GROUP BY email
  HAVING COUNT(*) > 1
);

This query uses a subquery to get the list of duplicate emails and then uses the IN operator to select all the rows that have those emails.

Another approach to finding duplicates is to use the ROW_NUMBER() function to assign a unique number to each row and then filter for rows that have a number greater than 1. Here’s an example:

SELECT *
FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY email ORDER BY id) as rn
  FROM customers
) t
WHERE rn > 1;

This query assigns a row number to each row based on the email column and orders them by the id column. The outer query then filters for rows with a row number greater than 1, which are the duplicate rows.

In conclusion, finding duplicate values in SQL Server is an important task in data management. By using the GROUP BY and HAVING clauses or the ROW_NUMBER() function, you can easily identify and remove duplicates from your tables.