Try this.
select q.source, q.target, q.robot, q.origin, q.probe, q.qos, s.sampletime from s_qos_snapshot s inner join s_Qos_data q on s.table_id=q.table_id GROUP BY s.table_id, q.source, q.target, q.robot, q.origin, q.probe, q.qos, s.sampletime having datediff(hour, max(s.sampletime), getdate()) > 48