Should Top N SQL Queries Include Duplicate Maximum Values?

Finding Top N Highest Values in MySQL Tables

In data analysis and reporting, it is often necessary to retrieve the top records based on the highest values of a specific column. However, ambiguity can arise when multiple records share the same maximum value near the Nth position.

Question:

When using SQL to select the top N rows with the highest values for a particular column, should the query return only the top N rows or include additional rows with the same maximum value?

Answer:

The answer depends on whether you want to include or exclude duplicate top values. Here are two approaches:

Approach 1: Exclude Duplicate Top Values

To retrieve only the top N rows without duplicates, use the following query:

SELECT *
FROM t
ORDER BY value DESC
LIMIT N

In this query, the LIMIT N clause restricts the result set to the top N rows, excluding any rows with the same maximum value below the Nth row.

Approach 2: Include Duplicate Top Values

To retrieve all rows with the highest value, regardless of duplicates, use the following query:

SELECT *
FROM t JOIN
(SELECT min(value) AS cutoff
 FROM (SELECT value
       FROM t
       ORDER BY value
       LIMIT N
      ) tlim
) tlim
ON t.value >= tlim.cutoff;

This query uses a subquery to determine the cutoff value for rows with the Nth highest value. It then joins this subquery with the main table using an ON clause to filter for rows with values greater than or equal to the cutoff.

Additional Considerations:

• If multiple rows have the same maximum value and the Nth row is not a duplicate, Approach 1 will return only the top N rows.
• Approach 2 will always return all rows with the Nth highest value, regardless of whether it is the result of duplicates or not.
• The performance of the query may vary depending on the table size and indexing.

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