How to group product by category?

The CodeIgniter Active Record Class offers a method called group_by which you can use to query a table with the results representing a grouping of the records in the table.

Code:

$this->db->select("COUNT(id)");
$this->db->group_by("role");
$this->db->get("my_table");

You should probably read the mysql documentation on 'group by'. Or if you are using some other DBMS, check out its documentation on group by.