sql - PostGIS: syntax-basics for function

I am a completely new in PostGIS.

I tried to solve a problem I had in QGIS (see Simplify multiple lines: create central axis?).

I just need to know how the correct syntax works to use the function ST_ApproximateMedialAxis(geometry geom);

I succesfully added the geometry to the pgAdmin4 Geometry Viewer with the query

select geom from public.bufferrail; - so that seems to be correct.

But how to include this geometry as argument to the function?

SELECT ST_ApproximateMedialAxis (select geom from public.bufferrail); does not work. I tried several variations, with no success.

Answer

The correct query is Select uid, ST_ApproximateMedialAxis(geom) from public.bufferrail;