Determine date overlap.
 
Suppose you have a table with a start_date and end_date. If you have the requirement to determine whether there is overlap in the periods defined by the start_data and end_dates you can use the following approach.
 
With the following query overlap can be determined. If rows are returned, periods are in overlap.

select  *
from    my_table a
,       my_table b
where   b.start_date < a.end_date
and     b.end_date   > a.start_date
and not b.rowid = a.rowid

In this query we make a cartesian product of the rows in the table. With the condition "not b.rowid = a.rowid" we ensure that a row is compared with itself. What remains is a combination of all the other possible rows.