-- not what you want
SELECT service_id, min(distance) FROM table
GROUP BY service_id;

##</code><code>##

SELECT stop_reference, service_id, distance FROM table
ORDER BY service_id, distance;

##</code><code>##

my $last_service_id = -8675309;    # for a good time, call
while ($sth->fetch()) {
    if ($service != $last_service_id) {
        print "min dist for service '$service' = '$distance' at stop '$stops'\n";
        $last_service_id = $service;
    }
}