Adding sortable columns to Users Table in Admin section

<th class="left"-->
    <!--?php echo JHtml::_('grid.sort', 'Candidate Number', 'candidate_number', $listDirn, $listOrder); ?-->
</th>

Adding sortable columns to a table in a component

Step 1=> Model

protected function populateState($ordering = null, $direction = null) {
    parent::populateState('default_column_name', 'ASC');
}

Replace 'default_sort_column' with the column we want to sort by default.
We can also change 'ASC' to 'DESC', depending on which way we want to sort by default.

public function __construct($config = array())
    {   
        $config['filter_fields'] = array(          
            'candidate_number',         
            'username'
        );
        parent::__construct($config);
    }

public function getListQuery() {
        $db = JFactory::getDbo();
        $query = $db->getQuery(true);  
        // ...                  
        // Add the list ordering clause in last.
        $query->order($db->escape($this->getState('list.ordering', 'default_sort_column')).' '.$db->escape($this->getState('list.direction', 'ASC')));
        return $query;

    }

Step 2: The View

Now we will assign sortable columns to the view so that they show up on the page when it is displayed. Add following code to view.html.php file.

public function display($tpl=null) {
    $items = $this->get('Items');
    $state = $this->get('State');

    $this->sortDirection = $state->get('list.direction');
    $this->sortColumn = $state->get('list.ordering');

    parent::display($tpl);
}

Step 3: The Template

grid.sort: so that JHTML will insert the correct behaviour for a sortable column.
Candidate Number: Name of column that is displayed to the user on table.
candidate_number: Name of datbase field.
$this->sortDirection: Current order direction.
$this->sortColumn: Currently sorted column name.

<form action="index.php?option=com_users" method="post" name="adminForm">    
    <table class="adminlist" cellpadding="1">                   
        <th class="title">
        <?php echo JHTML::_('grid.sort',   'Candidate Number', 'candidate_number', $this->sortDirection, $this->sortColumn ); ?>
        </th>                           
        <td>
            <?php echo $row->candidate_number; ?>
        </td>
    </table>
</form>


<script language="javascript" type="text/javascript">
function tableOrdering( order, dir, task )
{
    var form = document.adminForm;

    form.filter_order.value = order;
    form.filter_order_Dir.value = dir;
    document.adminForm.submit( task );
}
</script>

Reference

https://docs.joomla.org/Adding_sortable_columns_to_atable_in_acomponent