Best Way to Delete Duplicate Rows in SQL: Detailed Guide

Duplicate records in SQL databases are a common issue that can slow down query execution. Human error, or some system glitches, are the reasons for duplicates in databases. Delete duplicate rows in SQL is important for database optimization and reliability. In this blog, we will discuss the process and resolve it with multiple SQL queries. Moreover, we are going to use the SQL Recovery Tool to ease this process of removing duplicate records.
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Necessity to Remove Duplicates in SQL

Deleting duplicates from the database is crucial for maintaining accuracy and efficiency.

• Duplicate database entries occupy extra storage space, which impacts database efficiency.
• They introduce inconsistencies that can distort query outcomes and reports.
• Unwanted duplicates make it challenging to maintain clean and reliable datasets.
• Analysis accuracy declines when similar rows appear multiple times.
• Poor data quality from duplicates can mislead business insights and strategy.

Methods to Delete Duplicate Rows in SQL

You can remove duplicate rows from SQL tables manually using a few reliable methods. Here are the most common and effective ones explained simply:

1. Using ROW_NUMBER() with CTE

This is one of the safest ways to delete duplicate records in MySQL while keeping one original record.

WITH DuplicateCTE AS (

SELECT *,

ROW_NUMBER() OVER (PARTITION BY Email ORDER BY EmployeeID) AS rn

FROM Employees

)

DELETE FROM DuplicateCTE

WHERE rn > 1;

2. Using GROUP BY and MIN()

You can delete duplicates by grouping similar records and keeping one instance of each.

DELETE FROM Employees

WHERE EmployeeID NOT IN (

SELECT MIN(EmployeeID)

FROM Employees

 GROUP BY Email

);

3. Using a Temporary Table

In this method, you can delete duplicate records in SQL without primary key.

By just copying unique data into a new table and dropping the old one.

-- Create a temporary table with unique rows

SELECT DISTINCT *

INTO temp_Employees

FROM Employees;


-- Drop the original table

DROP TABLE Employees;


-- Rename the temporary table to the original table name

EXEC sp_rename 'temp_Employees', 'Employees';

4. Using DELETE JOIN (for MySQL)

If you’re using MySQL, you can join the table with itself to delete duplicates.

DELETE t1

FROM Customers t1

JOIN Customers t2

ON t1.Email = t2.Email

WHERE t1.CustomerID > t2.CustomerID;

Pro Tip:

Before running any delete command, always take a backup of your database or run a SELECT query first to confirm which rows are being removed.

Best Way to Delete Duplicate Rows in SQL

All of the methods mentioned above are useful ways of removing duplicate records from SQL tables. Nevertheless, you are encouraged not to execute those queries against your live data. Instead, always run a test on a sample or a backup of your data first, for safety.

If you still are unable to get the duplicate records deleted in SQL Server 2000 or other versions, it is possible your database is corrupt. Corruption of the SQL database files can result in data inconsistency, duplicates, and general performance issues.

To overcome this, the DatabaseFileRecovery offers you the advanced SQL Recovery Tool. This tool is built to repair damaged SQL database files (MDF and NDF) and retrieve corrupted data efficiently. It ensures that your data is restored accurately and can even recover records that were accidentally deleted or lost, providing a reliable way to maintain database integrity.

Conclusion

It is important to delete duplicate rows in SQL because it can lead to data inaccuracies, slow performance, and unreliable results. It makes it crucial to address and remove them. Manual methods are effective for eliminating duplicates but should be applied carefully and tested on sample data. Moreover, by utilizing a professional SQL Recovery Tool, your corrupt MDF and NDF files will be repaired.

Frequently Asked Questions

About The Author:

Robert Scott

With more than five years of experience in email migration, Data Recovery, Email Backup, and File Management, I combine my years of experience with a strong interest in new technology. My professional journey is fueled by a genuine passion for navigating and mastering the latest advancements in these fields, ensuring that I stay ahead of the curve and bring innovative solutions to the table.

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