DQL Select every rows having one column's MAX value

问题

Working with Symfony 2 and Doctrine, I'm searching for a way to select every rows having the max value in a specific column.

Right now, I'm doing it in two queries:

• One to get the max value of the column in the table
• Then I select rows having this value.

I'm sure this can be done with one query.

Searching, I have found this answer in a thread, that seems to be what I am searching for, but in SQL.

So according to the answer's first solution, the query I'm trying to build would be something like that:

select yt.id, yt.rev, yt.contents
from YourTable yt
inner join(
    select id, max(rev) rev
    from YourTable
    group by id
) ss on yt.id = ss.id and yt.rev = ss.rev

Does anybody know how to make it in Doctrine DQL?

For now, here is the code for my tests (not working):

$qb2= $this->createQueryBuilder('ms')
            ->select('ms, MAX(m.periodeComptable) maxPeriode')
            ->where('ms.affaire = :affaire')
            ->setParameter('affaire', $affaire);

$qb = $this->createQueryBuilder('m')
            ->select('m')
            //->where('m.periodeComptable = maxPeriode')

            // This is what I thought was the most logical way of doing it:
            ->innerJoin('GAAffairesBundle:MontantMarche mm, MAX(mm.periodeComptable) maxPeriode', 'mm', 'WITH', 'm.periodeComptable = mm.maxPeriode')

            // This is a version trying with another query ($qb2) as subquery, which would be the better way of doing it for me,
            // as I am already using this subquery elsewhere
            //->innerJoin($qb2->getDQL(), 'sub', 'WITH', 'm.periodeComptable = sub.maxPeriode')

            // Another weird try mixing DQL and SQL logic :/
            //->innerJoin('SELECT MontantMarche mm, MAX(mm.periodeComptable) maxPeriode ON m.periodeComptable = mm.maxPeriode', 'sub')

            //->groupBy('m')
            ->andWhere('m.affaire = :affaire')
            ->setParameter('affaire', $affaire);

    return $qb->getQuery()->getResult();

The Entity is GAAffairesBundle:MontantMarche, so this code is in a method of the corresponding repository.

More generally, I'm learning about how to handle sub-queries (SQL & DQL) and DQL syntax for advanced queries.

Thx!

回答1:

After some hours of headache and googling and stackOverflow readings... I finally found out how to make it.

Here is my final DQL queryBuilder code:

    $qb = $this->createQueryBuilder('a');
    $qb2= $this->createQueryBuilder('mss')
            ->select('MAX(mss.periodeComptable) maxPeriode')
            ->where('mss.affaire = a')
            ;

    $qb ->innerJoin('GAAffairesBundle:MontantMarche', 'm', 'WITH', $qb->expr()->eq( 'm.periodeComptable', '('.$qb2->getDQL().')' ))
        ->where('a = :affaire')
        ->setParameter('affaire', $affaire)
        ;

    return $qb->getQuery()->getResult();

回答2:

For me when i trying to make a subquery i make:

->andWhere($qb->expr()->eq('affaire', $qb2->getDql()));

回答3:

To achieve this using pure DQL and without use of any aggregate function you can write doctrine query as

SELECT a
FROM GAAffairesBundle:MontantMarche a
    LEFT JOIN GAAffairesBundle:MontantMarche b
    WITH a.affaire = b.affaire 
    AND a.periodeComptable < b.periodeComptable
WHERE b.affaire IS NULL
ORDER BY a.periodeComptable DESC

The above will return you max record per group (per affaire)

Expalnation

The equivalent SQL for above DQL will be like

SELECT a.*
FROM MontantMarche a
LEFT JOIN MontantMarche b 
    ON a.affaire = b.affaire 
    AND a.periodeComptable < b.periodeComptable
WHERE b.affaire IS NULL
ORDER BY a.periodeComptable DESC

Here i assume there can be multiple entries in table e.g(MontantMarche) for each affaire, so here i am trying to do a self join on affaire and another tweak in join is i am trying to join only rows from right table(b) where a's periodeComptable < b's periodeComptable, So the row for left table (a) with highest periodeComptable will have a null row from right table(b) thus to pick the highest row per affaire the WHERE right table row IS NULL necessary.

Similarly using your posted sample query with inner join can be written as

select yt.id, yt.rev, yt.contents
from YourTable yt
left join YourTable ss on yt.id = ss.id and yt.rev < ss.rev
where ss.rev is null

Hope it makes sense

来源：https://stackoverflow.com/questions/29962725/dql-select-every-rows-having-one-columns-max-value