[aposi]

Hola, prueba con un left join

Código SQL [-]
Select v.Mes, sum(v.Total) as Total, sum(d.cantidad) as Cantidad
from VentasABC v 
left join DetalleABC d on d.ID_Venta=v.ID_Venta 
Where d.ID_Concepto in (101,102) 
group by v.mes