How to get max() of integer value from a varchar field in codeigniter?

rf_id  rf_name  
12      12
13      13
14      14
15      15
16      GF
17      BASE

I want to get 15 from column rf_name in Codeigniter. each time I should get max('rf_name') where characters not allowed

First, make sure column rf_name data type should be integer/float, not varchar/string/text etc.

So, according to the data type of the column, it will not store any string value

$maxid = $this->db->query('SELECT MAX(rf_name) AS maxid FROM table')->row()->maxid;

OR

$this->db->select_max('rf_name');
$query = $this->db->get('table');

If you have both string and numeric values you have to do the following steps

1. Get all values of column

   $this->db->select('rf_name');
   $this-db->from('table');
   $values = $this->db->get('table')->result_array(); 
   

2. Get only values from an array

   $only_values= array();
   foreach($values as $value){
      if (is_numeric($value['rf_name'])) {
         array_push($only_values, $value['rf_name']);
      }
   }
   

3. use max() and pass the array of values to get maximum numeric value from it.

   $max_value = max($only_values);