Unexpected duplication in mysqli/php

Question

I'm using MySQLi with PHP(5.2.4) MySQL(5.0.51a) on Ubuntu 8.04 LAMP.

The relevant Db tables are below:

Questions
+----+------------------------------------------------------------------------------------------------------------+
| id | question                                                                                                   |
+----+------------------------------------------------------------------------------------------------------------+
|  1 | What is the correct rate of flow (in millilitres per minute) for a 14<abbr title="Gauge">G</abbr> cannula? |
|  2 | Which of the following drugs is not an anaesthetic  induction agent?                                       | 
+----+------------------------------------------------------------------------------------------------------------+

answers
+----+--------------------------------------------------------+
| id | answer                                                 |
+----+--------------------------------------------------------+
|  1 | 344<abbr title="millilitres per minute">ml/min</abbr>. | 
|  2 | 205<abbr title="millilitres per minute">ml/min</abbr>. | 
|  3 | 98<abbr title="millilitres per minute">ml/min</abbr>.  | 
|  4 | 60<abbr title="millilitres per minute">ml/min</abbr>.  | 
|  5 | Thiopental sodium                                      | 
|  6 | Propofol                                               | 
|  7 | Etomidate                                              | 
|  8 | Domperidone                                            | 
+----+--------------------------------------------------------+

a_lookup (to associate questions with answers)
+------+------+
| q_id | a_id |
+------+------+
|    1 |    1 | 
|    1 |    2 | 
|    1 |    3 | 
|    1 |    4 | 
|    2 |    5 | 
|    2 |    6 | 
|    2 |    7 | 
|    2 |    8 | 
+------+------+

I'm connecting to the Db with MySQLi, using the following (bearing in mind that I just edited out the real username, password and database name in favour of place-holder generics):

<?php

if (!isset($qtype) && !isset($submitted)) {

/*
=========================================================================
=   queries the database if no specific question-type selected =
=========================================================================
*/


    $mysqli = new mysqli("localhost", "username", "password", "db_name");

    if ($result = $mysqli->query(
       "SELECT questions.id as qid, 
        questions.question as question
       FROM
        questions;")) {


     while ($row = $result->fetch_object()) {

      $i = $row->qid;

      $q[$i][question] = $row->question;

      if ($answers = $mysqli->query(
         "SELECT answers.id, answers.answer FROM answers, a_lookup, questions WHERE answers.id=a_lookup.a_id AND '$i'=a_lookup.q_id;")) {

       while ($row = $answers->fetch_object()) {

        if (!isset($c)) {
         $c = 1;
        }
        else {
         $c = $c;
        }

        $q[$i][answers][$c] = $row->answer;

         $c++;
       }

      }


     }


    }

$mysqli->close();



}

elseif (isset($qtype)) {

// this part should hopefully be invoked when tags 'question-types' are supported

}

?>

This returns the questions as expected, the print_r is below:

Array
(
    [1] => Array
        (
            [question] => What is the correct rate of flow (in millilitres per minute) for a 14G cannula?
            [answers] => Array
                (
                    [1] => 344ml/min.
                    [2] => 205ml/min.
                    [3] => 98ml/min.
                    [4] => 60ml/min.
                    [5] => 344ml/min.
                    [6] => 205ml/min.
                    [7] => 98ml/min.
                    [8] => 60ml/min.
                )

        )

    [2] => Array
        (
            [question] => Which of the following drugs is not an anaesthetic  induction agent?
            [answers] => Array
                (
                    [9] => Thiopental sodium
                    [10] => Propofol
                    [11] => Etomidate
                    [12] => Domperidone
                    [13] => Thiopental sodium
                    [14] => Propofol
                    [15] => Etomidate
                    [16] => Domperidone
                )

        )

I'm just confused by my ending up with eight results, rather than the four I was expecting. I get the eight -as anyone that knows anything about databases would expect whether I use the mysql terminal client, or php's mysql-api.

What am I doing wrong to create the duplicates?

Answer 1

You shouldn't really run queries in a loop like that because it will be very inefficient - you shoud be able to fetch the data in one query, for example:

SELECT Questions.id asquestion_id, Questions.question, answers.id as answer_id, answers.answer
FROM Questions
INNER JOIN a_lookup ON (Questions.id = a_lookup.q_id)
INNER JOIN answers ON (a_lookup.a_id = answers.id)

It should be straightfoward to sort the results from that into the same structure and it should be much quicker, e.g.

$result = array();
while ($row = $result->fetch_assoc()) {
    $questionId = $row['question_id'];
    if (!isset($result[$questionId])) {
        $result[$questionId] = array('question'=>$result['question'], 'answers' => array());
    }
    $result[$questionId]['answers'][] = $row['answer'];
}

Answer 2

You get duplicate answers because your second sql query matches for either question id in the questions table:

Try

 SELECT answers.id, answers.answer FROM answers, a_lookup, questions WHERE
 answers.id=a_lookup.a_id AND '$i'=a_lookup.q_id AND '$i' = questions.id;

instead

In any case. your multiple tables in FROM is not very easy to follow. Try to JOIN instead because then you do not forget the filter in a table since you have to give one in every join.