How to create a mysqli dynamic WHERE clause

Question

I have an application here: Application

The application is there to show you the two drop down menus I want to use as a filter to be able to get Student's Answers determining on the student(s) selected and the question(s) selected form the respective drop down menus.

To see the drop down menus, in the application select an Assessment from the Assessment drop down menu and submit, you will see the students and questions drop down menus displayed underneath.

Now what I want to do is to create a dynamic WHERE clause depending on the options selected from the student(s) and question(s) drop down menus when the user clicked on the Get Student Answers button.

Below is the current query. The query has to have the default clause SessionId = ? at all times in the WHERE clause. The other clauses studentId = ? and questionId = ? depends on the options the user has chose from both drop downs.

$selectedstudentanswerqry = "
SELECT
StudentAlias, q.SessionId, QuestionNo, QuestionContent, o.OptionType, GROUP_CONCAT( DISTINCT Answer
ORDER BY Answer SEPARATOR ',' ) AS Answer, r.ReplyType, 
GROUP_CONCAT(DISTINCT StudentAnswer ORDER BY StudentAnswer SEPARATOR ',') AS StudentAnswer, ResponseTime
FROM Student s
INNER JOIN Student_Answer sa ON (s.StudentId = sa.StudentId)
INNER JOIN Student_Response sr ON (sa.StudentId = sr.StudentId)
INNER JOIN Question q ON (sa.QuestionId = q.QuestionId)
INNER JOIN Answer an ON q.QuestionId = an.QuestionId
LEFT JOIN Reply r ON q.ReplyId = r.ReplyId
LEFT JOIN Option_Table o ON q.OptionId = o.OptionId
WHERE (SessionId = ?)
GROUP BY sa.StudentId, q.QuestionId
ORDER BY StudentAlias, q.SessionId, QuestionNo
";

global $mysqli;
$selectedstudentanswerstmt=$mysqli->prepare($selectedstudentanswerqry);
// You only need to call bind_param once
$selectedstudentanswerstmt->bind_param("i",$_POST["session"]);
// get result and assign variables (prefix with db)
$selectedstudentanswerstmt->execute(); 
$selectedstudentanswerstmt->bind_result($detailsStudentAlias,$detailsSessionId,$detailsQuestionNo, 
$detailsQuestonContent,$detailsOptionType,$detailsAnswer,$detailsReplyType,$detailsStudentAnswer,$detailsResponseTime);
$selectedstudentanswerstmt->store_result();
$selectedstudentanswernum = $selectedstudentanswerstmt->num_rows(); 

Below is the Students and Questions drop down menus as a sample html:

Students Drop Down Menu:

<select name="student" id="studentsDrop">
<option value="All">All</option>
<option value="3">u0499220 - Jack Briggs</option>
<option value="7">u0093220 - Mary Kay</option>
</form>

Questions Drop Down Menu:

<select name="question" id="questionsDrop">
<option value="All">All</option>
<option value="34">1</option>
<option value="35">2</option>
<option value="36">3</option>
</form>

I am thinking of something like if a specific student is selected then include StudentId = ? in WHERE clause, if specific question number is selected then include QuestionId = ? in WHERE clause. But if the All value is selected in Student drop down menu then remove StudentId = ? from the WHERE clause as we are look for all student, not whittling down for a specific student. This is same for if All value is selected from Question drop down menu but obviously dealing with QuestionId = ?

Answer 1

Did you know that you can write a subquery here?

WHERE (SessionId = ?)

like

WHERE SessionId IN(SELECT SessionId FROM ...)

http://dev.mysql.com/tech-resources/articles/subqueries_part_1.html

Answer 2

Ok, you know what? I think I have misunderstood the real question here. You may find this to work better:

    $selectedstudentanswerqry = "
    SELECT
    StudentAlias, q.SessionId, QuestionNo, QuestionContent, o.OptionType, GROUP_CONCAT( DISTINCT Answer
    ORDER BY Answer SEPARATOR ',' ) AS Answer, r.ReplyType, 
    GROUP_CONCAT(DISTINCT StudentAnswer ORDER BY StudentAnswer SEPARATOR ',') AS StudentAnswer, ResponseTime
    FROM Student s
    INNER JOIN Student_Answer sa ON (s.StudentId = sa.StudentId)
    INNER JOIN Student_Response sr ON (sa.StudentId = sr.StudentId)
    INNER JOIN Question q ON (sa.QuestionId = q.QuestionId)
    INNER JOIN Answer an ON q.QuestionId = an.QuestionId
    LEFT JOIN Reply r ON q.ReplyId = r.ReplyId
    LEFT JOIN Option_Table o ON q.OptionId = o.OptionId";

    if (studentId = something && questionId = something) {

    $selectedstudentanswerqry .= "WHERE (SessionId = ?)
    GROUP BY sa.StudentId, q.QuestionId
    ORDER BY StudentAlias, q.SessionId, QuestionNo
    ";

    } else if (studentId = somethingelse && questionId = somethingelse) {

    $selectedstudentanswerqry .= "WHERE (SessionId = ?)
    GROUP BY sa.StudentId, q.QuestionId
    ORDER BY StudentAlias, q.SessionId, QuestionNo
    ";

    } else {
    $selectedstudentanswerqry .= "WHERE (SessionId = ?)
    GROUP BY sa.StudentId, q.QuestionId
    ORDER BY StudentAlias, q.SessionId, QuestionNo
    ";
    }