Boss: guard rule
Facility bookings are measured in slots. One facility may hold at most 16 slots per calendar day in total. A trigger must stop the booking that would break the limit. Four hidden checks run your trigger.
task
A facility may be booked for at most 16 slots a day in total. Write a BEFORE INSERT trigger on bookings that raises SQLSTATE 23514 when a new booking would exceed it. Four hidden inserts test it.