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this is my error page.the value of quantity was caluclated wrong.

this is my picture one,the table name is salesitem.

this is my picture two,the table name is purchaseitem.
this is my picture three,the table name is opstock.

 public function St_statement()


 $startdate = $this->input->post('SDate');
 $enddate = $this->input->post('EDate');
 $date = str_replace('/', '-', $startdate);
 $newDate = date("Y-m-d", strtotime($date));
 $date2 = str_replace('/', '-', $enddate);
 $newDate2 = date("Y-m-d", strtotime($date2));
 $data['startdate'] = $startdate;
 $data['enddate'] = $enddate;
 if ($this->input->post('all'))
 
 $this->db->where('billdate >=', $newDate);
 $this->db->where('billdate <=', $newDate2);

 $this->db->select("Prdtname");
 $this->db->select_sum("qty");
 $this->db->select_sum("Quantity");
 $this->db->from('purchaseitem');
 $this->db->group_by("Prdtname");
 [![enter image description here][4]][4]




 $this->db->join('salesitem','salesitem.Product_Code = purchaseitem.Product_Code','left outer');
 $query = $this->db->get('')->result_array();
 $data['query'] = $query;

 $this->load->view('Inventory/St_Stmt', $data);
 



 this is my controller code....... 

in this picture the 2 tables are successfully joined but the sum of quantity is wronged.
please help me to caluculate the sum of quantity from three table using codeiginter