仅返回一个数据AJAX

Here's what i'm doing. Im trying to return a data through my database and i want the data to display in an organized manner. I need to distribute the data in different html tables based on what year or semester the data is. If you know about curriculum it's like that.

here's my algorithm. first i initialize a variable outside the while loop which is $a, if semester = 1st and year = 1 then it will go through another if which is checking if $a = 1 if it's equals it will create a div that contains part of the html tables then it will print out the data fetched from the database in td after that it will check again if $a = 1 then it will create the last part of the html tables.

To summarize. I want to distribute the data based on what semester and year the data is. But the problem is only one data is shown i have two data which equals to 1st semester and year 1.

Data

    $a = 1;
    while($row = mysqli_fetch_array($result))
    {
        if($row['semester'] == '1st' && $row['year'] == 1){

           /*This if condition below is set to initiliaze the table headers only once*/

            if($a == 1){
                $output .='<div class="table-responsive">
                           <table class="table table-striped">
                            <thead>
                              <tr>
                                  <th class="text-center">Subject Code</th>
                                  <th class="text-center">Subject Title</th>
                                  <th class="text-center">Units</th>
                                  <th class="text-center">Pre-req</th>
                                  <th class="text-center">Co-req</th>
                              </tr>
                          </thead>
                      <tbody>';
              }

                $output .= '<tr>
                         <td class="text-center">'.$row['code'].'</td>
                         <td class="text-center">'.$row['title'].'</td>
                         <td class="text-center">'.$row['unit'].'</td>
                         <td class="text-center">'.$row['prereq'].'</td>
                         <td class="text-center">'.$row['corequisite'].'</td>
                      </tr>';


   /*This if condition below is set to initiliaze the table headers only once*/

              if($a == 1){
                $output .= '</tbody>
                            </table>
                          </div';
                $a = 2;
              }

        }

    }
   echo $output;