como obtener diferencias en acumulados

[novatosql]

yo he realizado la siguiente consulta:

select week(dia_planing,3) as semana,
case when week(dia_planing,3)=(select (week(dia_planing,3)) from reservas order by dia_planing) then numero_reservas
else (numero_reservas-(select numero_reservas from reservas as reservasanteriores where week(dia_planing,3)=(week(dia_planing,3)-1))) end as unidades
from reservas order by dia_planing

esto consulta me da subquery returns more than 1 row