How to search and sorting in datatable with pagination using ajax?

<?php
defined('BASEPATH') OR exit('No direct script access allowed');
class Search_controller extends CI_Controller
{

public function __construct()
{
parent::__construct();
$this->load->model('Common');
}

public function index()
{
$this->load->view('custom-filter-search');
}
public function get_employeeList()
{
$postData = $this->input->post();
$data = $this->Common->getEmployees_list($postData);
echo json_encode($data);
}


}

function getEmployees_list($postData=null)
{

$response = array();

## Read value
$draw = $postData['draw'];
$start = $postData['start'];
$rowperpage = $postData['length']; // Rows display per page
$columnIndex = $postData['order'][0]['column']; // Column index
$columnName = $postData['columns'][$columnIndex]['data']; // Column name
$columnSortOrder = $postData['order'][0]['dir']; // asc or desc
$searchValue = $postData['search']['value']; // Search value

## Search
$searchQuery = "";
if($searchValue != ''){
$searchQuery = " (file_no like '%".$searchValue."%' or case_no like '%".$searchValue."%' or complaint_officer like'%".$searchValue."%' ) ";
}

## Total number of records without filtering
$this->db->select('count(*) as allcount');
$records = $this->db->get('employees')->result();
$totalRecords = $records[0]->allcount;

## Total number of record with filtering
$this->db->select('count(*) as allcount');
if($searchQuery != '')
$this->db->where($searchQuery);
$records = $this->db->get('employees')->result();
$totalRecordwithFilter = $records[0]->allcount;

## Fetch records
$this->db->select('*');
if($searchQuery != '')
$this->db->where($searchQuery);
$this->db->order_by($columnName, $columnSortOrder);
$this->db->limit($rowperpage, $start);
$records = $this->db->get('employees')->result();

$data = array();

foreach($records as $record ){

$data[] = array(
"id"=> $record->id,
"file_no"=>$record->file_no,
"case_no"=>$record->case_no,
//"type_of_cases"=>$record->type_of_cases,
"type_of_cases"=>$this->get_case_name($record->type_of_cases),
"name_of_court"=>$this->get_court_name($record->name_of_court),
"complaint_officer"=>$record->complaint_officer,
"name_of_accused"=>$record->name_of_accused,
"date_of_order"=>changeDateFormat('d-m-Y',$record->date_of_order),
"created_at"=>changeDateFormat('d-m-Y',$record->created_at)


);
}

## Response
$response = array(
"draw" => intval($draw),
"iTotalRecords" => $totalRecords,
"iTotalDisplayRecords" => $totalRecordwithFilter,
"aaData" => $data
);

return $response;
}


public function get_case_name($id)
{
$this->db->select('type_of_cases');
$this->db->from('cases_type');
$this->db->where('id',$id);
$query=$this->db->get();
//$this->db->last_query();

$kk = $query->result();


return $kk[0]->type_of_cases;

}

public function get_court_name($name_of_court)
{
if($name_of_court == '1')
{
$nameofcourt = "Supream Court";
}
elseif($name_of_court == '2')
{
$nameofcourt = "High Court";
}
elseif($name_of_court == '3'){
$nameofcourt = "Civil Court";
}

return $nameofcourt;
}

<!-- Datatable CSS -->
<link href='//cdn.datatables.net/1.10.19/css/jquery.dataTables.min.css' rel='stylesheet' type='text/css'>

<!-- jQuery Library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<!-- Datatable JS -->
<script src="//cdn.datatables.net/1.10.19/js/jquery.dataTables.min.js"></script> <table class="table table-striped table-bordered mou display dataTable" id="empTable">
<thead>
<tr style="">
<th>S.No</b></th>
<th>File No</th>
<th>Case No.</th>
<th>Type Of Cases</th>
<th>Name Of Court</th>
<th>Complaint Officer</th>
<th>Name of accused</th>
<th>Case File Date</th>
<th>Received Date</th>
<th>Action</th>
</tr>
</thead>

</table>





<!-- Script -->
<script type="text/javascript">
$(document).ready(function(){
$('#empTable').DataTable({
'processing': true,
'serverSide': true,
'serverMethod': 'post',
'ajax': {
'url':'<?=base_url()?>Search_controller/get_employeeList'
},
'columns': [
{ "data": null, "render": function (data, type, full, meta) {
return meta.row + 1;
}
},
{ data: 'file_no', "autoWidth": true },
{ data: 'case_no' },
{ data: 'type_of_cases' },
{ data: 'name_of_court' },
{ data: 'complaint_officer' },
{ data: 'name_of_accused' },
{ data: 'date_of_order' },
{ data: 'created_at' },
{
"render": function (data, type, full, meta) {
var kk = full.id;
return '<a class="btn btn-info" href="<?php echo base_url()?>court/courtcases/case_view/'+full.id+'">View</a>';
}
},
]
});
});
</script>