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