news category and subcategory

i have table CAT if parent_id is set to zero its main if not its subcategory
cat_id | name |parent_id
1 Bob 0
2 John 1
3 Mike 1



and table NEWS

id | cat_id | title | text
1 1 text some text
2 2 any any text
3 3 another another text


my url structure is ..mysite/page/cat_id

question is if i click on link BOB whos id is 1 i want to get all data from NEWS table with title (text, any, another)
and if i click on link John whos id is 2 i want to get all data from NEWS table with title (any)
and if i click on link Mike whos id is 3 i want to get all data from NEWS table with title (another)


below is code i write only gives me news set to its cat_id not working for main category bob.

PHP Code:

$this->db->select('*');      
         $this->db->where('cat_id', $cat_id);       
         $query = $this->db->get("news");
       return $query->result();