How to Efficiently Retrieve the Maximum Row per Group in Doctrine DQL?

Doctrine Query Language: Retrieving Maximum or Latest Row per Group

In Doctrine Query Language (DQL), retrieving the maximum or latest row per group can be achieved using subqueries and joins.

Problem Statement:

The original SQL statement attempts to retrieve the maximum score per name using an inner join to a subquery. However, the DQL translation in the question exhibits a syntax error, indicating that an association named "name" does not exist in the "Score" entity.

Solution:

The suggested solution avoids utilizing aggregate functions and focuses on using joins to exclude lower-scoring rows. This approach ensures efficient querying and eliminates the need for additional logic or calculations.

DQL Equivalent:

SELECT a 
FROM AppBundle\Entity\Score a
    LEFT JOIN AppBundle\Entity\Score b 
    WITH a.name = b.name AND a.score < b.score
WHERE b.score IS NULL
ORDER BY a.score DESC

Explanation:

The left join condition ensures that only rows with the highest score per name are included in the result. Rows with a lower score will be filtered out by the subsequent WHERE clause, which checks for the absence of a higher score for the same name.

Query Builder Alternative:

This approach can also be implemented using the Query Builder:

$repo = $DM->getRepository('AppBundle\Entity\Score');
$results = $repo->createQueryBuilder('a')
                ->select('a')
                ->leftJoin(
                    'AppBundle\Entity\Score',
                    'b',
                    'WITH',
                    'a.name = b.name AND a.score < b.score'
                )
                ->where('b.score IS NULL')
                ->orderBy('a.score', 'DESC')
                ->getQuery()
                ->getResult();

Additional Considerations:

An alternative approach involves creating a database view that represents the desired query results. This view can then be mapped to an entity in Doctrine, providing a seamless way to access the data without the need for complex queries. However, this approach is generally discouraged as it may lead to performance and maintenance issues.

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