Codeigniter where not exists does not work

Why would you not want to use a JOIN? It would certainly be faster than a subquery.

You could probably do something like this (untested):

PHP Code:

$result = $this->db->select('POST.ID, POST.DATE, TYPE.Post_type AS P_TYPE, (SELECT ID FROM CATEGORY WHERE CATEGORY.POST_ID = POST.ID) AS CID', false)
  ->join('TYPE', 'POST.TYPE_ID = TYPE.ID', 'left')
  ->where('POST.ID', $id)
  ->where_in('TYPE.Post_Type', ['type_1', 'type_2'])
  ->order_by('POST.ID', 'DESC')
  ->having('CID IS NOT NULL', false)
  ->get('POST');