Question

I got my table called cars , and the second table called uploads which has a car_id field, I can LEFT JOIN this, so the uploads.car_id = cars.id, the problem comes, and uploads table, for more than one record for certain car id, what then? I have to use second query in a loop like this?

$cars = $PDO->query("SELECT * FROM `cars`")->fetchAll();
foreach($cars as $car)
{
    $uploads = $PDO->query("SELECT * from uploads WHERE id = " . (int)$car['id'])->fetch();
    echo 'My upload file name:' . $uploads['hash'];
}

? Its not a good idea, because $cars query may return more than 500 records at once… So it will be a huge performance lost.

So my question is, what can I do in this case, to prevent the query in the loop?

Answer 1

You can use the GROUP BY and GROUB_CONCAT sql functions to get what you want in one query with better performance

try :

$cars = $PDO->query("SELECT cars.id as carID, GROUP_CONCAT(hash) AS hashs FROM uploads left join cars on uploads.car_id = cars.id group by cars.id ")->fetchAll();

foreach($cars as $car)
{
   echo 'Car id '.$car['carID'].'  --  uploaded files name:' . $uploads['hashs'];
}