check whether any date ranges for a patient overlap

[ask77]

I work on health care claims and right now I have a task to determine if any date ranges for a single patient overlap and I need to count the number of overlapping days for each date range for that patient. Here is the snap shot of the data. Also for the overlapping days I need to calculate the total dosage and also calculate the average dose for those overlapping days. Any help is greatly appreciated.

overlapping date ranges

[Kurt_Bremser]

Use the lag() function to retrieve the enddate of the previous observation, and compare that with the filldate of the current obs.