Please show the full query. It may be possible to modify the query, itself, to do what you ask.

If modifying the query to do all the work isn't possible, and presuming service_id is the route number, I would loop through the data, filling an array indexed by service_id. When the distance is less than the existing distance, replace it with the distance just read. Filling in your while loop with some guaranteed-to-look-like-code pseudo-code:

my @stop_list;
my @distance;
while($sth -> fetch()){
    if((not defined($distance[$service]) or
       $distance[$service] > $distance) {

         $stop_list[$service] = $stops;
         $distance[$service] = $distance;
     }
}
foreach $service (@stop_list) {
   print "Stop: $stop_list[$service] distance: $distance[$service]\n"
}
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Note that this is absolutely untested; it's pseudo-code.

This will set $stop_list[$service] to the appropriate stop number and $distance[$service] to the appropriate $stops and $distance, respectively, if the current value of $distance[$service] is undefined or greater than the distance just returned by the fetch.

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