FIND_IN_SET() how to implement in mysql and codeigniter

I Have two table in Database here i put my two table and i want record like Ex> Suppose if user 1 is login and user category_id is 1,2,3 then i want record from 2nd table like Home,Marketing,Customer so how to fire query FIND_IN_SET() in mysql and Codeigniter

1st* user_registration
userid  roll_id   fullname    username          password               category_id
  1      1          admin     [email protected]   admin              1,2,3
  2      2         user         [email protected]        user             1,2,3,4,5,6

2nd*Category
category_id    cname         parentID      

 1                  Home                  0          
 2                Marketing              0             
 3                Customer              2             
 4                Quoation               2                 
 5            New Customer          3         
 6           view Customer           3