How to Efficiently Delete All But the Top N Rows in a SQL Table?

Deleting All but Top n Rows in a SQL Table

Often, we encounter scenarios where we need to retain only a specific number of top rows from a database table while deleting the rest. There are efficient ways to accomplish this using SQL.

Solution:

One of the most effective methods to delete all but the top n rows from a table is to leverage a subquery. The following query demonstrates how:

DELETE FROM Table WHERE ID NOT IN (SELECT TOP 10 ID FROM Table)

This query identifies the IDs of the top n rows based on a descending order of the ID column using the TOP keyword. It then excludes these rows from deletion by comparing the IDs against the subquery results.

Performance Considerations:

It's important to note a potential performance hit with using this approach. Chris has highlighted that the TOP 10 query will be executed for each row being deleted, which can be inefficient if there are a large number of rows.

For one-time operations, this may not pose a significant issue. However, for frequent executions, there are alternative optimizations to consider.

Remember, the specific approach and optimizations may vary depending on the database management system and the underlying data characteristics.

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