difficult JOIN

Hi,

i have the following SQL-Problem:

We are using 2 tables. The first, called plan, is holding planned
working times for employees per
tour:

plan.id_tour
plan.id_employee
plan.begin_time
plan.end_time

The second table 'work' stores the actual worked times for employees per tour:

work.id_tour
work.id_employee
work.begin_time
work.end_time

Employees can be multiple times assigned to one tour. One record will
be created for every
assignment. They can also work multiple times in one tour.

Now i wanna merge this infos into one report. I wanna join the first
plan entry for one employee in
one tour with the first work entry for one employee in one tour and so on.

How can i obtain that? A simply USING(id_tour, id_employee) -JOIN will not doit.

Thanks for any hints,
Thomas