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.

Partitioning a table in SQL Server that has a primary key referenced by foreign keys in other tables

 Partitioning a table in SQL Server that has a primary key referenced by foreign keys in other tables can indeed be challenging due to the constraints involved. Your approach of dropping and recreating constraints is a common solution, but here are a few considerations that might simplify the process or make it more efficient:

1. Use SWITCH Instead of Dropping and Recreating Constraints:

If you are partitioning for performance reasons, consider using the SWITCH statement. You can create a new partitioned table with the desired schema, and then use ALTER TABLE SWITCH to move data between tables. This avoids dropping foreign key constraints but requires more setup and might need the table to be empty when switching.

• Step-by-Step:
  1. Create a new partitioned table with the same schema as the original table.
  2. Use INSERT INTO ... SELECT to move data to the new table.
  3. Use ALTER TABLE SWITCH to switch the tables.
  4. Drop the old table and rename the new one.

2. Temporarily Disable Constraints:

SQL Server allows you to disable foreign key constraints temporarily, which might help in avoiding the need to drop them.

• Step-by-Step:

  1. Disable the foreign key constraints on the dependent tables.
  2. Drop the primary key constraint and clustered index.
  3. Partition the table and recreate the primary key constraint on the partitioned table.
  4. Re-enable the foreign key constraints.

• Example:

   
  ALTER TABLE [DependentTable] NOCHECK CONSTRAINT [FK_Name];
  -- Drop and recreate the primary key and clustered index
  ALTER TABLE [DependentTable] CHECK CONSTRAINT [FK_Name];
  

3. Using Schema Modification with Minimal Downtime:

If downtime is a concern, consider using techniques like online index creation and schema modification that might reduce the impact on the application.

• Online Index Creation: SQL Server Enterprise Edition supports creating and rebuilding indexes online, which might reduce the impact during partitioning.
• Schema Modifications: You could stage the new partitioned table while keeping the original table intact, then switch over with minimal downtime.

4. Consideration for SQL Server Version:

If you're using SQL Server 2016 or later, take advantage of improvements in partitioning and index creation features, like support for more efficient operations on partitioned tables.

5. Using a Maintenance Window:

Since the process involves significant changes, performing this operation during a maintenance window might be the best option, even if it means temporarily disabling or dropping constraints.

6. Documentation and Backup:

Document each step carefully and ensure you have a full backup before proceeding. This will help in case anything goes wrong during the process.

Conclusion:

Unfortunately, there isn’t a way to completely avoid the process of dropping and recreating constraints when partitioning a table that is heavily referenced by foreign keys. However, depending on your specific environment and requirements, the alternatives like SWITCH, temporarily disabling constraints, or using online operations might make the process smoother and less disruptive.