MySql Find multiple rows for each id - php

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...

Answer 2

The problem is in using the = operator and passing a set of values instead of single one. Change your query to the following and try again
SELECT * FROM questions q1
WHERE cid
IN $cids
ORDER BY q1.qid LIMIT 20

Answer 3

The following snippet uses the MySQL variable trick to assign a number for each row per CID. To keep the example simple I've limited the amount of returned rows to 2 per CID.
select cid
, qid
, text
from (
select if(#last_cid = cid, #rn := #rn + 1, #rn := 1) as rn
, (#last_cid := cid)
, cid
, qid
, text
from YourTable yt
cross join
(select #rn := 0, #last_cid := -1) r
) as SubQueryAlias
where rn < 3;
Data setup:
create table YourTable (QID int, TEXT varchar(50), CID int);
insert YourTable values
(1, 'hi', 1),
(1, 'hi', 1),
(2, 'hi', 1),
(2, 'hi', 1),
(3, 'hi', 2),
(4, 'hi', 2),
(4, 'hi', 2),
(5, 'hi', 3);
This returns up to two rows per CID:
+------+------+------+
| cid | qid | text |
+------+------+------+
| 1 | 1 | hi |
| 1 | 1 | hi |
| 2 | 3 | hi |
| 2 | 4 | hi |
| 3 | 5 | hi |
+------+------+------+