Retrieving Rows with Maximum Values per Group in MySQL

In many database applications, you need to retrieve rows that have the maximum value of a specific column for each group of rows. This can be achieved using various techniques in MySQL. In this tutorial, we will explore how to write efficient queries to solve this problem.

Understanding the Problem

Suppose we have a table called TopTen with columns id, home, datetime, player, and resource. We want to retrieve the rows for each distinct home value that has the maximum datetime value. This means we need to group the rows by the home column and find the row with the maximum datetime value for each group.

Using a Subquery

One way to solve this problem is to use a subquery to find the maximum datetime value for each home group, and then join the original table with the subquery result. Here’s an example query:

SELECT tt.*
FROM TopTen tt
INNER JOIN (
  SELECT home, MAX(datetime) AS MaxDateTime
  FROM TopTen
  GROUP BY home
) groupedtt 
ON tt.home = groupedtt.home 
AND tt.datetime = groupedtt.MaxDateTime;

This query works by first finding the maximum datetime value for each home group using a subquery. The result of the subquery is then joined with the original table on both the home and datetime columns.

Using a Self-Join

Another way to solve this problem is to use a self-join, where we join the table with itself on the home column and filter out rows that have a smaller datetime value. Here’s an example query:

SELECT m.* 
FROM TopTen m
LEFT JOIN TopTen b
  ON m.home = b.home AND m.datetime < b.datetime
WHERE b.datetime IS NULL;

This query works by joining the table with itself on the home column and filtering out rows that have a smaller datetime value. The result is a set of rows that have the maximum datetime value for each home group.

Using Window Functions

If you are using MySQL 8 or later, you can use window functions to solve this problem more efficiently. Here’s an example query:

SELECT id, home, datetime, player, resource 
FROM (
  SELECT id, home, datetime, player, resource,
    RANK() OVER (PARTITION BY home ORDER BY datetime DESC) AS rank
  FROM TopTen
) t
WHERE rank = 1;

This query works by using the RANK window function to assign a ranking to each row within each home group based on the datetime value. The result is then filtered to only include rows with a ranking of 1, which corresponds to the maximum datetime value for each home group.

Conclusion

In this tutorial, we explored three different techniques for retrieving rows with maximum values per group in MySQL: using a subquery, using a self-join, and using window functions. Each technique has its own advantages and disadvantages, and the choice of which one to use depends on the specific requirements of your application.