Getting the output of a function used in a where clause

HI,

I’m a newbie so please bear with me. I have a function defined (got it from one of your threads… thanks Joe Conway) which calculates the distance  between 2 zip code centeroids (in lat,long). This thing works great. However, I want to sort my results by distance without incurring the additional burden of executing the function twice. A simplified version of my current SQL (written in a perl cgi)  that returns a set of zip codes within a given radius is:

$sql = “SELECT zipcode from zipcodes where zipdist($lat1d,$lon1d,lat,long) <= $dist;”;

What I want to write is something like:

$sql = “SELECT zipcode, distance from zipcodes where distance <= $dist order by distance;”;

But I don’t the magic SQL phrase to populate the distance variable using my nifty function. Do I need to create an output type for distance?

Thanks in advance!

Bill