Removing duplicate rows from a large table with millions of records

 Removing duplicate rows from a large table with millions of records can be challenging, but it can be done efficiently using SQL. Here's a step-by-step guide to remove duplicates from a table in SQL Server:

1. Identify Duplicates:

First, you'll need to identify what constitutes a "duplicate." Typically, this means all columns (except for the primary key or a unique identifier) are the same.

2. Create a Backup:

Before making any changes, it's good practice to create a backup of your table.

 
SELECT * INTO YourTable_Backup
FROM YourTable;

3. Remove Duplicates Using a CTE (Common Table Expression):

The most common and efficient way to remove duplicates is to use a CTE combined with the ROW_NUMBER() function. Here's how you can do it:

 
WITH CTE AS (
    SELECT 
        *,
        ROW_NUMBER() OVER (PARTITION BY Column1, Column2, Column3 ORDER BY (SELECT NULL)) AS RN
    FROM 
        YourTable
)
DELETE FROM CTE
WHERE RN > 1;

• Explanation:
  • The ROW_NUMBER() function assigns a unique sequential integer to rows within a partition of a result set, starting at 1 for the first row in each partition.
  • The PARTITION BY clause divides the result set into partitions to which the ROW_NUMBER() function is applied.
  • The ORDER BY (SELECT NULL) clause simply ensures that the rows are arbitrarily ordered within each partition.
  • Rows with RN > 1 are considered duplicates and are deleted.

4. Verify the Results:

After running the deletion, it's essential to verify that duplicates have been removed and that the table integrity is maintained.

 
SELECT Column1, Column2, Column3, COUNT(*)
FROM YourTable
GROUP BY Column1, Column2, Column3
HAVING COUNT(*) > 1;

This query should return no results if all duplicates have been successfully removed.

5. Rebuild Indexes and Update Statistics:

After removing duplicates, it's a good idea to rebuild any indexes on the table and update the statistics.

 
ALTER INDEX ALL ON YourTable REBUILD;
UPDATE STATISTICS YourTable;

6. Considerations for Large Tables:

For very large tables (like your 260 million rows), you might need to remove duplicates in smaller batches to avoid long transaction times or transaction log growth. This can be done using a loop or by deleting rows in chunks.

Example:

 
WHILE EXISTS (SELECT 1 FROM CTE WHERE RN > 1)
BEGIN
    DELETE TOP (10000) FROM CTE WHERE RN > 1;
END

7. Monitor and Optimize:

Monitor the process and optimize as needed, particularly in a production environment where long-running queries could impact performance.

Conclusion:

By following these steps, you should be able to efficiently remove duplicate rows from your large table in SQL Server. The key is to use a method that minimizes the impact on your server and ensures the integrity of your data.