How to structure effectively news and category relationship ?

If you just want to join all articles to the associated category you just need a quey like that.

PHP Code:

$this->db->from('category')
     ->join('news', 'category.cat_id = news.cat_id')
     ->where('category.cat_id', $cat_id)
     ->get(); 


cat_id is the matching key for both tables.