toggle a value in database based on user input of check box

I have a list of items that will be fetched from DB. Each row have one checkbox and an associated value in DB. If it is checked and saved the value changes to 1 else it will be 0.

So on page load there is a drop down to select the items.Based on the selection, the data will be fetched. Values with 1 will be shown checked and 0 will be unchecked. I am trying to update this to DB on button click. So newly checked ones will be saved with status 1 and unchecked will be updated with status 0.

If one value was previously 1 and is unchecked during update, the value should change to 0.

My code:

HTML:

// output data of each row
while($row = $result->fetch_assoc()) {
   ?>

                    <tbody>
                      <tr class="even pointer" id="<?php echo $row['tcode'] ?>">
                        <td class="a-center ">
                          <?php if($row['selected'] == 0){ ?>
                          <input class="flat" type="checkbox" name="check_list[]" value="<?php echo $row['tcode'] ?>">
                        <?php } else{ ?>
                          <input class="flat" type="checkbox" name="check_list[]" value="<?php echo $row['tcode'] ?>" checked>
                        <?php }  ?>
                        </td>
                        <td class=" "><?php echo $row['task_name'] ?></td>
                        <td class=" "><?php echo $row['tcode'] ?></td>
                        <td class=" "><?php echo $row['task_category'] ?></td>
                        </td>
                      </tr>

                    </tbody>
                <?php } ?>

PHP:

if(isset($_POST['submit'])){//to run PHP script on submit
if(!empty($_POST['check_list'])){
    $projectcode=$_POST['projectcategory'];
// Loop to store and display values of individual checked checkbox.
    echo $_POST['check_list'];
foreach($_POST['check_list'] as $selected){
$sql2="UPDATE mappedtask SET selected=1 WHERE pcode = '$projectcode' AND pcode = '.$_POST[tcode].' ";
$result2=$conn->query($sql2);
}
}
}