Add union() to DB Query Builder

@skunkbad

Code:

-- Filling a dropdown menu about citizenship
(SELECT 'XX' AS country_code, '-- Not specified' AS country_name)
UNION
-- The most probable option(s) is moved on top for user convenience
(SELECT country_code, country_name FROM countries
WHERE country_code IN ('US')
ORDER BY country_name ASC)
UNION
(SELECT country_code, country_name FROM countries
WHERE country_code NOT IN ('US')
ORDER BY country_name ASC)