Add union() to DB Query Builder

UNION is just a tool and it is programmer's responsibility to decide where and why to use it.

db->union(): I can imagine something else (CI3-like syntax):

Code:

$result = $this->db
    ->union_start()
        ->select("'XX' AS country_code, '-- Not specified --' AS country_name")
    ->union_end()
    ->union_start()
        ->select("country_code, country_name")
        ->from('countries')
        ->where_in('country_code', array('US'))
        ->order_by('country_name', 'asc')
    ->union_end()
    ->union_start()
        ->select("country_code, country_name")
        ->from('countries')
        ->where_not_in('country_code', array('US'))
        ->order_by('country_name', 'asc')
    ->union_end()
    // For other cases: A place for clauses that affect the whole result: ORDER BY, LIMIT, etc.
    ->get()
    ->result_array();

The only problem is how hard the implementation of UNION would be.