We do similar queries. We have "facilites" that may change all of part of their definitions and when they are changed we enter an effective date. This allows us to track a range of data based on the history ( effective ) date. I assume you could do the same by first querying to get the record ids corresponding to the latest 20 visits to the physician ( or the dates if you don't use surrogate keys ) for that criteria and then in your main query join back into the first query with an inner join on those record ids ( or dates ) to limit your result set to only the records you want based on the date range.