prueba con esta consulta:
Código:
SELECT v.Mes, sum(v.Total) AS Total, sum(d.cantidad) AS Cantidad
FROM VentasABC v
LEFT JOIN (SELECT ID_Venta, sum(cantidad) AS cantidad
FROM DetalleABC
WHERE ID_Concepto IN (101, 102)) d
ON d.ID_Venta = v.ID_Venta
GROUP BY v.mes