Retrieving the Last Record in Each Group - MySQL

Mysql Sql Group By Greatest N Per Group Groupwise Maximum

When working with MySQL, there are often situations where we need to retrieve the last record in each group based on a certain criteria. This can be a challenging task, but there are several approaches we can take to solve this problem efficiently.

Understanding the Problem

Let's take a look at the example provided:

            
            Id   Name   Other_Columns
            -------------------------
            1    A       A_data_1
            2    A       A_data_2
            3    A       A_data_3
            4    B       B_data_1
            5    B       B_data_2
            6    C       C_data_1

If we run the query SELECT * FROM messages GROUP BY name, we will get the result as:

            
            1    A       A_data_1
            4    B       B_data_1
            6    C       C_data_1

However, we want to retrieve the last record in each group, which would result in the following:

            
            3    A       A_data_3
            5    B       B_data_2
            6    C       C_data_1

Approach 1: Subquery with ORDER BY

One approach to solve this problem is to use a subquery with an ORDER BY clause. Here is the query:

            
            SELECT * FROM (SELECT * FROM messages ORDER BY id DESC) AS x GROUP BY name

This query works by first sorting the records in descending order based on the primary key, and then grouping them by name. The first record in each group will be the last record based on the ordering in the subquery. While this approach might seem inefficient, MySQL is optimized to handle this type of query efficiently.

Approach 2: Self-Join

Another approach is to use a self-join to get the last record in each group. Here is the query:

            
            SELECT m1.*
            FROM messages m1
            LEFT JOIN messages m2 ON m1.name = m2.name AND m1.id < m2.id
            WHERE m2.id IS NULL
  • In this query, we join the messages table with itself on the condition that the name is the same and the ID of the first record is less than the ID of the second record.
  • The LEFT JOIN ensures that we get all records from the first table (m1) and any matching records from the second table (m2).
  • The WHERE clause filters out any records where the second table's ID is not null. This means that we only get the last record in each group.

Conclusion

Retrieving the last record in each group in MySQL can be achieved using different approaches. Whether you choose to use a subquery with an ORDER BY clause or a self-join, it's important to consider the efficiency of your query and the size of your dataset. You can test both approaches on your own data and determine which one works best for your specific scenario.

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