Please imagine this small database...

Diagram

Tables

Volunteer     Event         Shift         EventVolunteer
=========     =====         =====         ==============
Id            Id            Id            EventId
Name          Name          EventId       VolunteerId
Email         Location      VolunteerId
Phone         Day           Description
Comment       Description   Start
                            End

Associations

Volunteers may sign up for multiple events.
Events may be staffed by multiple volunteers.

An event may have multiple shifts.
A shift belongs to only a single event.

A shift may be staffed by only a single volunteer.
A volunteer may staff multiple shifts.

Check Constraints

1. Can I create a check constraint to enforce that no shift is staffed by a volunteer that's not signed up for that shift's event?

2. Can I create a check constraint to enforce that two overlapping shifts are never staffed by the same volunteer?