Question

I have a table (in MS Access) that stores in each record the start and end date for one event associated with an item. Multiple events may be associated with each item, and event time periods may overlap.

I will use the term ‘open event’ to mean an event that has a start date less than and an end date greater than a given date.

I would like a query that gives me the list of items that had at least one open event within a given time period, but which also have no open events at the end of the time period. Ideally, I would also like to list for each of these items the id of the last open event in the time period.

Here is a list of event records that represent some cases I need to have covered. The time period in question is 2012-03-20 to 2012-03-30:

eventId   itemId   startDate    endDate
e1        i1       2012-03-21   2012-03-23   -- event open entirely inside of time period
e2        i2       2012-03-19   2012-03-21   -- event open at start date
e3        i3       2012-03-29   2012-03-31   -- event open at end date

e4        i4       2012-03-19   2012-03-26   -- multi-event item with event open at end date
e5        i4       2012-03-22   2012-03-25
e6        i4       2012-03-29   2012-03-31
e7        i4       2012-04-01   2012-04-30

e8        i5       2012-03-19   2012-03-25   -- multi-event item with no events open at end date
e9        i5       2012-03-22   2012-03-29
e10       i5       2012-03-25   2012-03-26
e11       i5       2012-04-01   2012-04-30

e12       i6       2012-03-13   2012-03-19   -- event not in time period at all

And here are the items (and their last-open events) that I’d like to see as a result of this query:

i1, e1
i2, e2
i5, e9  -- note that e9.endDate > e8.endDate and e10.endDate, and that e11 falls after the time period in question, so is not considered the last event for the item

Answer 1

You could left join to a subquery that lists all events that are open at the enddate. If you demand that a column from the subquery is null, you exclude all event/item combinations that match the subquery.

select  distinct e.eventId
,       e.itemId
from    Events e
left join
        (
        select  distinct itemId
        from    Events
        where   startDate < '2012-03-30' -- Started before end
                and '2012-03-30' < EndDate -- Closed after end
        ) oe
on      e.itemId = oe.itemId
where   e.startDate < '2012-03-30' -- Started before end
        and '2012-03-20' < e.EndDate -- Ended after start
        and oe.eventId is null -- Not open at end