Finding duplicate rows in SQL Server

sql

duplicates

inner-join

deletion

byNikita Barsukov·Aug 8, 2024

Quickly identify duplicates in SQL Server by simply using GROUP BY on your column(s) of interest and filtering duplicates with HAVING COUNT(*) > 1:

SELECT column_name, COUNT(*)
FROM table_name
GROUP BY column_name
HAVING COUNT(*) > 1;

This single-line magic lists all duplicate values within your desired column_name alongside their occurrence counts.

Expanded answer: Nifty techniques to find and manage duplicates

Extra information on duplicates with subqueries and joins

Ever needed to bag more details like the IDs or carry out more intricate operations on duplicates? Nothing that a pair of INNER JOIN and a subquery can't handle!

Method 1: ID listing for duplicate rows with an INNER JOIN

Your duplicate row IDs are just an INNER JOIN away:

SELECT a.ID, a.column_name, COUNT(*)
FROM table_name AS a
INNER JOIN (SELECT column_name FROM table_name GROUP BY column_name HAVING COUNT(*) > 1) AS b
ON a.column_name = b.column_name
GROUP BY a.column_name, a.ID;

As your DBA pal would say, "Joins, like coffee, make everything better!" ;)

This query does the heavy lifting of attaching each duplicate column_name with its corresponding row IDs like a name tag on a party guest!

Window of opportunity: Applying window functions for duplicate tagging

ROW_NUMBER(), a window function, can add a personal touch to your duplicate management strategies:

WITH DuplicateRanks AS (
  SELECT *, ROW_NUMBER() OVER(PARTITION BY column_name ORDER BY (SELECT NULL)) AS Rank
  FROM table_name
)
SELECT *
FROM DuplicateRanks
WHERE Rank > 1;

This query generates a unique sequence (Our matching T-shirts at a party?) to all rows sharing a common value, making it easier to discern between the originals and the clones.

De-duplication process: Ensuring impact-free deletion

Retaining data sanctity during duplicate removal

Any operation with DELETE in it should sound as scary as it can get. Therefore, before removing duplicates, data safety is a ONE PRIORITY you don't want to mess with!

When handling duplicates, you need to make sure you don't accidentally delete unique data. Let's avoid those "Oops! Deleted my database" T-shirts!

Deleting duplicates: The RIGHT way

When we talk about deletion, it's all about removing all but one of the duplicate occurrences or deleting based on a condition using a MAX or MIN function:

WITH CTE AS (
    SELECT *, ROW_NUMBER() OVER(PARTITION BY column_name ORDER BY some_criteria) AS rn
    FROM table_name
)
DELETE FROM CTE WHERE rn > 1;

"DELETE FROM. You won't be missed." 😉👋

After running the delete operation, remember to check if you need to do any cleanup by running the duplicate detection query. "Better safe than sorry," eh?

Duplicates with connected information need to be treated carefully. Like when you have multiple party guests (users) linked to a party host (organization), consolidate this data before showing any of them the exit door.

The accuracy test: ensuring every operation is spot-on

Double-checking before deletion

Before you hit that "Delete" button, cross-verify your list of duplicates to ensure you only remove the intended data. As they say, "Measure twice, cut once."

Being adaptive to your data's story

Every database has a unique structure and specific set of demands, so you would need to fine-tune these queries to work best with your SQL Server database.