Filter by column alias (firebird sql)

Question

I made a UDF called DistanceMeters which returns the distance between two coordinates as an integer. Now I want to use it to filter the results, for example,

SELECT DistanceMeters("Latitude_C_", "Longitude_C_", 44.894115, -123.031717) as dis 
FROM CURR_LOCATION where dis < 5000 ORDER by dis

The problems is that I can't refer to the column alias in the where condition:

SQL error code = -206
Column unknown
DIS

This will work:

SELECT * FROM CURR_LOCATION where DistanceMeters("Latitude_C_", "Longitude_C_", 44.894115, -123.031717) < 5000

but I want to show the distance as a result too.

Help?

Answer 1

One option would be to use derived table:

SELECT dis FROM
  (SELECT DistanceMeters("Latitude_C_", "Longitude_C_", 44.894115, -123.031717) as dis 
   FROM CURR_LOCATION)
WHERE dis < 5000 ORDER BY dis