Delete All But Top N Rows in SQL

Understanding the Task:

• All other rows should be deleted.
• You want to retain only the top n rows (where n is a specific number) based on a specific column's values.
• You have a database table with multiple rows.

General SQL Approach:

1. Identify the Table and Column:

2. Create a Temporary Table:

3. Insert Top n Rows into Temporary Table:

4. Delete Rows from Original Table:

   • Use a DELETE statement to remove all rows from the original table.

5. Repopulate Original Table with Top n Rows:

   • Insert the rows from the temporary table back into the original table.

SQL Example:

Assuming you have a table named products with a column sales_quantity, and you want to keep the top 10 products with the highest sales:

CREATE TEMPORARY TABLE top_10_products
SELECT product_id, product_name, sales_quantity
FROM products
ORDER BY sales_quantity DESC
LIMIT 10;

DELETE FROM products;

INSERT INTO products
SELECT *
FROM top_10_products;

DROP TABLE top_10_products;

Explanation:

1. Create temporary table: Creates a temporary table top_10_products to store the top 10 products.
2. Insert top n rows: Selects the top 10 products based on sales_quantity in descending order and inserts them into top_10_products.
3. Delete rows: Deletes all rows from the original products table.
4. Repopulate original table: Inserts the top 10 products from the temporary table back into the original products table.
5. Drop temporary table: Removes the temporary table.

Additional Considerations:

• Data Integrity: Ensure that deleting rows doesn't violate any data integrity constraints (e.g., foreign key relationships).
• Performance: For large tables, consider using indexes on the column used for ordering to improve performance.
• Handling Ties: If multiple products have the same sales quantity and are among the top n, you might need to consider additional criteria or use a ROW_NUMBER() function to handle ties.

CREATE TEMPORARY TABLE top_10_products
SELECT product_id, product_name, sales_quantity
FROM products
ORDER BY sales_quantity DESC
LIMIT 10;

DELETE FROM products;

INSERT INTO products
SELECT *
FROM top_10_products;

DROP TABLE top_10_products;

Alternative Methods for Deleting All But Top N Rows in SQL

While the previous response outlined a common approach using a temporary table, here are some alternative methods that can be employed:

Using ROW_NUMBER() (Common Table Expression):

• Steps:
  1. Create a Common Table Expression (CTE) to assign row numbers.
  2. Use a DELETE statement to remove rows where the row number is greater than n.
• Purpose: Directly assigns a row number to each row based on an ORDER BY clause.

WITH RankedProducts AS (
    SELECT product_id, product_name, sales_quantity,
           ROW_NUMBER() OVER (ORDER BY sales_quantity DESC) AS RowNum
    FROM products
)
DELETE FROM RankedProducts
WHERE RowNum > 10;

Using a Subquery with NOT EXISTS:

• Steps:
  1. Create a subquery to find the top n values.
  2. Use a DELETE statement with NOT EXISTS to delete rows whose values are not in the subquery.
• Purpose: Checks if a row's value is among the top n values.

DELETE FROM products
WHERE NOT EXISTS (
    SELECT 1
    FROM (
        SELECT product_id
        FROM products
        ORDER BY sales_quantity DESC
        LIMIT 10
    ) AS Top10
    WHERE products.product_id = Top10.product_id
);

Using a TRUNCATE Statement (with Caution):

• Caution: This method is destructive and cannot be undone. Use with extreme care.
• Purpose: Directly truncates the table, keeping only the specified number of rows.

TRUNCATE TABLE products
KEEP 10 ROWS;

Choosing the Best Method: The most suitable method depends on factors like:

• Data integrity: Ensure that the method doesn't violate any data constraints or cause unintended side effects.
• Performance: Consider the performance implications of each method, especially for large datasets.
• Database system: Some systems may have specific optimizations for certain methods.