How to Efficiently Find the Maximum or Latest Row Per Group Using Doctrine Query Language?

Doctrine Query Language: Finding Maximum or Latest Rows Per Group

In some database operations, it is necessary to query for the maximum or latest row within a group of data. To translate SQL statements that perform such operations into Doctrine Query Language (DQL), you must leverage specific techniques.

One common SQL technique is to use a subquery to determine the highest or latest value within a group and then join the main query with those results. However, this approach can become complex when it comes to Doctrine.

An alternative solution is to rewrite the SQL statement to avoid using aggregate functions. For instance, to find the highest score per name, you can use the following SQL statement:

SELECT 
  a.* 
FROM
  score a 
  LEFT JOIN score b 
    ON a.name = b.name 
    AND a.score < b.score 
WHERE b.score IS NULL 
ORDER BY a.score DESC 

This query selects all rows from the score table where there is no higher score recorded for the same name.

To translate the SQL statement to DQL, you can write:

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

You can also use the query builder API in Doctrine to construct the query:

$DM   = $this->get( 'Doctrine' )->getManager();
$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();

This provides an efficient way to perform the same operation as the original SQL statement using Doctrine.

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