postgis - Using CASE to select between two geometry functions?

I'm trying to cut holes in forest polygons, where there are lakes.

My PostGIS/PostgreSQL code look like this:

CREATE TABLE samp.forest_with_holes AS
  SELECT
    fo.objectid,
    (CASE ST_Intersects(fo.geom, la.geom)
      WHEN TRUE THEN ST_Difference(fo.geom, so.geom)
      ELSE fo.geom

    END CASE;) as geom
  from samp.forest as fo, samp.lake as la;

But I get the error message:

ERROR:  syntax error at or near "CASE"
LINE xx:     END CASE;) as geom

Can anybody help me to see why this is not working - and please suggest a better way :-)

Answer

1. 
   

   Correct way of using CASE:

   
   

   CASE 
     WHEN ST_Intersects(fo.geom, la.geom) THEN ST_Difference(fo.geom, la.geom)
     ELSE fo.geom
   END as geom
   



2. 
   

   Better query:

   
   

   SELECT fo.objectid, COALESCE(ST_Difference(fo.geom, ST_Collect(la.geom)), fo.geom) AS geom
   
   FROM samp.forest AS fo 
   LEFT JOIN samp.lake AS la ON ST_Intersects(fo.geom, la.geom)
   GROUP BY fo.objectid

EDIT:

Coalesce gives you the first non-null input and since the difference between a geom and null results in null it will return the next value: 'fo.geom'. The ST_Collect aggregate is needed to make sure you cut multiple holes when there is more than 1 lake in the same forest polygon.