Points within km range

[Thomas Schlage]

Hey guys.

I probably got an simple question but I didn't found a satisfying answer.
In my application the user insert a center point (lat, lon) and a range in kilometers. Now I want to get all entries which are in range of the lat,lon + km.
The way to calculate the distance in km inside the query seems to be the worst ever since the logic applies to every entry comparison.

I tried to convert the kilometers to a distance by calculating a bounding box and using Pythagoras to get the distance between center and max lat,long. It seems to work but it isn't accurate.

I'm searching for something like
SELECT * FROM entry WHERE ST_Distance(location, POINT(13, 15)) < KmToDistance(11);
or
SELECT * FROM entry WHERE ST_Contains(CreateRange(POINT(13,15), 11), location)


Can someone help me?