method chaining

[eluser]praveenarya[/eluser]
hi all,
using method chaining system i have written like

Code:

$this->db->select('*')->from('tbl_orders')->where('status=0 group by ref_id order by id desc');

                  $query = $this->db->get();

but i am getting error
Unknown column 'status=0' in 'where clause'
the quotation should be 'status'=0 instead it is placing as above

thanks in advance