SQL - Trigger Hospital

CREATE OR REPLACE TRIGGER T_BIENVENIDO_HOSPITAL
AFTER INSERT ON HOSPITAL
FOR EACH ROW
DECLARE
  Loc_x NUMBER;
  Loc_y NUMBER;
  Loc_ciudad elemento.ciudad%TYPE;
BEGIN
  SELECT el.x INTO Loc_x FROM elemento el, hospital ho WHERE :NEW.id_elem = el.id_elem;
  SELECT el.y INTO Loc_y FROM elemento el, hospital ho WHERE :NEW.id_elem = el.id_elem;
  SELECT el.ciudad INTO Loc_ciudad FROM elemento el, hospital ho WHERE :NEW.id_elem = el.id_elem;

  UPDATE unidad SET porcentaje_salud = 100
  WHERE EXISTS (SELECT * FROM unidad uni, elemento el
                WHERE (abs(el.x - Loc_x) + abs(el.y - Loc_y)) <= 5
                AND el.id_elem = uni.id_elem);

  DBMS_OUTPUT.PUT_LINE('Hospital construido );

END;