how to fetch record in codeigniter where clause using json values

My question is in my table stored json encoded values like field Name : offer_promotion

 [{"name":"sample promotion","price":"1555","expdate":"2017-05-15","shortdesc":"test","longdesc":"test"}]

Now i want to write query for select records like, For eaxmple:

$this->db->where('offer_promotion', "name":"sample promotion");

How to write this query?