Question

I have a table with 3 columns

---QID---TEXT---CID---

I would like to find 20 rows(QID and TEXT) for each distinct CID. I have already prepared string $cid so that I can use WHERE IN statement.

SELECT * FROM questions q1
WHERE cid=(SELECT cid 
    FROM questions q2 
    WHERE q2.cid IN ($cids)
    GROUP BY q2.cid)
ORDER BY q1.qid LIMIT 20

Thank you!

Answer 1

Simple query:

$query = 'SELECT QID, TEXT FROM yourDb.yourTable WHERE CID = '.$cid;

or, if $cid is an array:

$query = 'SELECT QID, TEXT FROM yourDb.yourTable WHERE CID IN('.implode(',',$cid).')';

To get to the results:

$pdo = new PDO('mysql:host=yourDBServer','login','password');
if (!$stmt = $pdo->query($query))
{
    die('query failed');
}
$results = $stmt->fetchAll(PDO::FETCH_ASSOC);

For more info on what you can do with the PDO object, refer to the manual

A quick fix (but not a good one) might be:

$q = 'SELECT QID, TEXT FROM yourDB.yourTB WHERE CID = '.$cid.' LIMIT 20';

In the case of CID IN(1,2,3), I’m not sure if there’s a strait forward way of doing this. All I can think of is using unions. Mayby this page can help you out with that.

A fugly fix might also be to ORDER BY CID ASC, and insted of using fetchAll(), do this:

$query = 'SELECT CID,QID, TEXT FROM yourDb.yourTable WHERE CID IN('.implode(',',$cid).')';

//execute query, same as above: $stmt holds results
$results = array();
while($row = $stmt->fetch(PDO::FETCH_ASSOC))
{
    if (!is_array($results[$row['CID']))
    {
        $results[$row['CID']] = array();
    }
    if (count($results[$row['CID']]) < 20)
    {
        $results[$row['CID']][] = $row;
    }
}

This way, the $results array, will have a key for each CID that was found, and that key’s value will be an array of up to 20 records…