SELECT * FROM regions WHERE id NOT IN (SELECT region_id FROM cities)

SELECT * FROM regions LEFT JOIN cities on (regions.id=cities.region_id) 
WHERE cities.region_id is null

SELECT regions.id AS 'childless_parents'
FROM regions
LEFT JOIN cities
ON regions.id = cities.region_id
WHERE cities.region_id IS NULL;

SELECT * FROM regions WHERE regions.id NOT IN (SELECT regions.id FROM regions 
INNER JOIN cities ON regions.id = cities.region_id)