Ya lo solucioné. De esta manera me funciona.
Código:
Select Sum(BO.QUANTITY) as TOT_OBJECTS, BO.CATEGORYNO, BO.SUBCATEGORYNO,
C1.CODENAME as CATNAME, C2.CODENAME as SUBCATNAME
from BOOKINGS_OBJECTSTYPE BO
left join CATEGORIES C1
on BO.CATEGORYNO = C1.CATEGORYNO
and C1.SUBCATEGORYNO = 0
left join CATEGORIES C2
on BO.CATEGORYNO = C2.CATEGORYNO
and BO.SUBCATEGORYNO = C2.SUBCATEGORYNO
where BO.BOOKINGNO = 1
group by BO.CATEGORYNO, BO.SUBCATEGORYNO, C1.CODENAME, C2.CODENAME