Error : Trying to get property num_rows of non-object property in wampserver

[Blessing Banda]

i am trying to fetch the candidate exam details about exam from DB on wampserver and it is giving me this error,

( ! ) Notice: Trying to get property 'num_rows' of non-object in C:\wamp64\www\online-exam-portal\code\manage-exam\_export-to-csv.php on line 85

here is the code

" . $userID . "

";
//exit();

// Fetch user responses to questions
$sql = "SELECT
'Multi-choice' AS exam_type,
mcq.question AS question_text,
mcq.correct_answer AS correct_answer,
mcqr.response AS chosen_option,
mcqr.score AS marks
FROM
multi_choice_question mcq
INNER JOIN multi_choice_response mcqr ON mcq.exam_id = mcqr.exam_id
AND mcq.question_no = mcqr.question_no
WHERE
mcqr.assignee_id = '$userID'
AND mcq.exam_id = $examID

    UNION

    SELECT
        'Fill in the blank' AS exam\_type,
        fiq.question AS question\_text,
        fir.response AS chosen\_option,
        fir.score AS marks
    FROM
        fill\_in\_question fiq
        INNER JOIN fill\_in\_response fir ON fiq.exam\_id = fir.exam\_id
            AND fiq.question\_no = fir.question\_no
    WHERE
        fir.assignee\_id = '$userID'
        AND fiq.exam\_id = $examID

    UNION

    SELECT
        'Theory' AS exam\_type,
        tq.question AS question\_text,
        tr.response AS chosen\_option,
        tr.score AS marks
    FROM
        theory\_question tq
        INNER JOIN theory\_response tr ON tq.exam\_id = tr.exam\_id
            AND tq.question\_no

[Lost User]

Please read my reply to the poster of the question below: Re: WordPress database error: [You have an error in your SQL syntax;[^].