Question

I’m just starting to learn more advanced SQL along with PHP and I’m really struggling trying to find out how to query my database for a quiz I’m building.

Ultimately, I’m trying to return a json object with the following structure which gives me a list of questions and all possible answers as a multidimensional array:

{
    "questions":
        [
            {
                "question": "question text here",
                "answers":
                    [
                         { "answer": "answer text here", "points": 10 },
                         { "answer": "answer text here", "points": 20 },
                         { "answer": "answer text here", "points": 30 },
                         { "answer": "answer text here", "points": 40 }
                    ]
            },
            {
                "question": "question text here",
                "answers":
                    [
                         { "answer": "answer text here", "points": 10 },
                         { "answer": "answer text here", "points": 20 },
                         { "answer": "answer text here", "points": 30 },
                         { "answer": "answer text here", "points": 40 }
                    ]
            }
        ]
{

…from my mySQL tables of the following structure:

quiz

id | title
1  | quiz title here

quiz_question

id | quiz_id (FK) | question_text
1  |       1      | question text here
2  |       1      | question text here

quiz_answer

id | quiz_question_id (FK) | answer_text      | points
1  |            1          | answer text here |   10
2  |            1          | answer text here |   20
3  |            1          | answer text here |   30
4  |            1          | answer text here |   40

…with the following foreign keys:

quiz_question.quiz_id is FK to quiz.id
quiz_answer.quiz_question_id is FK to quiz_question.quiz_id

…using the following PHP (in it’s simplest form which is currently only returning my questions):

//query the db
$query = mysql_query("
    SELECT quiz_question.question_text
    FROM quiz_question
    JOIN quiz ON quiz.id = quiz_question.quiz_id
    WHERE quiz.id = 1;
");

$numrows = mysql_num_rows($query);
for ($i = 0; $i < $numrows; $i++) {
    $row = mysql_fetch_assoc($query);
    $quiz_data[$i] = array("question" => $row["question_text"]);
}

//echo JSON to page
$response = $_GET["jsoncallback"] . "(" . json_encode($quiz_data) . ")";
echo $response;

…and using jQuery’s $.getJSON() in my JavaScript which gets my a JSON formatted object from my PHP which gets me back the following:

[
    {"question":"question text here"},
    {"question":"question text here"}
]

So my question is, how can I write my SQL and PHP to create a multidimensional array like the very above instead of a single array like I’m currently getting back now? I need to figure out how to include the questions and all associated answers as a multidimensional array.

Answer 1

You can’t retrieve a multi-dimensional array purely with mysql (at least as far as I know). You will have to do some php processing. This doesn’t sound too crazy.

First, update your query to select answers at the same time by joining quiz_answers on quiz_questions using the question ID. Then, in your loop:

$quiz = array();
while ($row = mysql_fetch_assoc($result)) {
   // you don't need to check num_rows
   // fetch_assoc returns false after the last row, so you can do this
   // which is cleaner
   if (!isset($quiz[$row['question_id'])) {
      $quiz[$row['question_id']] = array(
         'question' => $row['question_text']
         , 'answers' => array()
      );
   }
   $quiz[$row['question_id']]['answers'][] = $row['answer_text'];
}
$full = json_encode(array('questions' => $quiz'));

This will give you the array you want after it’s json encoded.

Note that you will end up selecting the question text/id once per each answer, which is inefficient. You can use GROUP_CONCAT on the answers, but the above will still work almost identically, you just have to split the answer string.

I also suggest you use PDO or some other wrapper over mysql_*.