How to Retrieve the Maximum or Latest Row Per Group Using Doctrine Query Language (DQL)?

Doctrine Query Language: Retrieving the Maximum or Latest Row Per Group

In order to retrieve the maximum or latest row for each group using the Doctrine Query Language (DQL), it is necessary to leverage subqueries to identify the target rows. The following steps outline how to achieve this:

Original SQL Query:

SELECT a.*
FROM score a
INNER JOIN (
  SELECT name, MAX(score) AS highest
  FROM score
  GROUP BY name
) b
ON a.score = b.highest AND a.name = b.name
GROUP BY name
ORDER BY b.highest DESC, a.dateCreated DESC

Doctrine Query Language Equivalent:

First Attempt:

$kb = $em->createQuery(
    "SELECT a 
    FROM ShmupBundle:Score a
    INNER JOIN a.name ShmupBundle:Score b WITH a.score = b.score AND a.name = b.name GROUP BY b.name
    WHERE a.platform='keyboard'
    GROUP BY a.name
    ORDER BY b.score DESC, a.dateCreated DESC"
);

Error:

[Semantical Error] line 0, col 73 near 'ShmupBundle:Score': Error: Class ShmupBundleEntityScore has no association named name

Corrected Version:

$kb = $em->createQuery(
    "SELECT a 
    FROM ShmupBundle:Score a
    INNER JOIN ShmupBundle:Score b WITH a.score = b.score AND a.name = b.name GROUP BY a.name
    WHERE a.platform='keyboard'
    GROUP BY a.name
    ORDER BY b.score DESC, a.dateCreated DESC"
);

Explanation:

In the original SQL query, a subquery is used to calculate the maximum score for each name. This result is then joined with the main query to identify the rows with the highest scores.

To translate this into DQL, a similar approach is required. However, the a.name field does not exist as an association in the Score entity (as indicated by the error message). To fix this, the subquery is written as a separate entity join, with the condition that a.score and b.score are equal and that a.name and b.name are equal.

An alternative to this approach is to revise the SQL query to use the ROW_NUMBER() function, which can provide the same results without the need for a subquery. The DQL used for this method depends on the version of Doctrine being used. For example:

use Doctrine\ORM\Query\Expr\Func;

$query = $em->createQuery(
    'SELECT a
    FROM Score AS a
    WHERE FUNC("ROW_NUMBER()", a.name) = 1
    ORDER BY a.score DESC'
);

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