How to Retrieve the Top n Max Values from a Database Table?

Selecting Top Max Values from a Table

In database management, it can be necessary to retrieve the top n highest values from a table. To achieve this, there are several approaches available. Consider the following table:

column1  column2
   1       foo
   2       foo
   3       foo
   4       foo
   5       bar
   6       bar
   7       bar
   8       bar

For n=2, we want to obtain these results:

3    
4    
7    
8    

Approximation Using Group Max

An initial approach is to calculate the maximum value for each group:

SELECT max(column1) FROM table GROUP BY column2

However, this method only provides the top value for each group, not the top n values.

Union-Based Solution

For n=2, the following union-based query can be used:

SELECT max(column1) m
FROM table t
GROUP BY column2
UNION
SELECT max(column1) m
FROM table t
WHERE column1 NOT IN (SELECT max(column1)
                      WHERE column2 = t.column2)

This query selects the top values and then excludes duplicates by joining the second query, which finds the next highest value not already selected.

Rank Simulation

For any value of n, a more generic approach is to simulate the rank() function over partitions. One way to do this is:

SELECT t.*
FROM
   (SELECT grouper,
          (SELECT val
           FROM table li
           WHERE li.grouper = dlo.grouper
           ORDER BY
                 li.grouper, li.val DESC
           LIMIT 2,1) AS mid
   FROM
      (
      SELECT DISTINCT grouper
      FROM table
      ) dlo
   ) lo, table t
WHERE t.grouper = lo.grouper
      AND t.val > lo.mid

This query groups the values by the desired column and then finds the next highest value for each group within the specified limit (in this case, 2,1 for n=2).

By following these approaches, you can effectively select the top n max values from a given table, ensuring that duplicate values are eliminated and the correct number of records is returned.

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