使用PHP查询具有相同关键字的多个MySQL表（具有不同结构）

I'm a newbie to PHP and MySQL. Trying to make a common search field for my website, which searches 3 different tables with different structure with the same keyword. 3 tables are course, lecture, faq.

My code is something similar: `

<?php
if(isset($_POST['submit'])) {
    //connect to the database
    $mysqli = new MySQLi("localhost","root","password","database");

    $search = $mysqli->real_escape_string($_POST['search']);

    //Query the db
    $result = $mysqli->query("
    SELECT topic, course_id, lect_no, table_id FROM lecture WHERE topic LIKE '%$search%'
    UNION
    SELECT course_name, course_id, table_id FROM course WHERE course_name LIKE '%$search%'
    UNION
    SELECT faq_link, table_id FROM faq WHERE faq_key LIKE '%$search%'
    ");

    if($result->num_rows > 0) {
        while($rows = $result->fetch_assoc()) {
            if($result->table_id == 1) {
                $topic = $rows['topic'];
                $course_id = $rows['course_id'];
                $lect_no = $rows['lect_no'];
                ?>
                <a href="http://www.xyz.php?course=<?php echo $course_id; ?>&lect=<?php echo $lect_no; ?>"><?php echo $topic; ?></a>
                <?php           
            } else if ($result->table_id == 2) {
                $course_name = $rows['course_name'];
                $course_id  = $rows['course_id'];
                ?>
                <a href="http://www.xyz.php?course=<?php echo $course_id; ?>"><?php echo $couse_name; ?></a>
                <?php
            } else if($result->table_id = 3) {
                $faq_link = $rows['faq_link'];
                echo $faq_link;
            }   
        }
    } else {
        echo "No Results Found";
    }
}
?>

`

But unfortunately it only searches in first table(lecture) only, even there are matches in other tables also. I searched a lot but didn't found any solution.