challenging query

I have a visitlocation table that looks like

personid varchar2(10);
location varchar2(10);
startdate date;

I fill this table based on transactions that I get from another system. This table keeps track of all of the locations that a person visited and the start date/time of the visit. How would I query this table to find all of the people (personid) that were in a particular location during a particular time period (say 7/1/1999 - 7/15/1999). I also need to show an end date for each location (based on the start date of next location) that the person visited.

I initially had an enddate in this table and had a stored procedure to set the enddate, in real time as I received transactions from the another system, of the prior location whenever the individual moved to a new location. The problem is that it is possible that the system that is feeding my system, can send me transactions that are out of transaction order or can send me duplicate transactions. When I took this approach, my visitlocation table got totally screwed up.

I was hoping someone could offer advice as to how to query the visitlocation table if I only kept the startdate and my queries had to calculate and display the enddate.

Thanks!
Lisa Received on Thu Aug 19 1999 - 14:56:14 CDT