CREATE OR REPLACE TRIGGER "T_CONST_TIERRA"
BEFORE INSERT
ON ELEMENTO
FOR EACH ROW
 WHEN (NEW.tipo='C') DECLARE
  temp VARCHAR2(30);
BEGIN
  SELECT pos.terreno INTO temp
  FROM posicion pos
  WHERE (pos.x=:NEW.x) AND (pos.y=:NEW.y);
  IF temp <> 'T' THEN raise_application_error(-20100, 'Se debe construir sobre tierra');
  END IF;
END;