Problems defining a primary key !!

Hi ,
i'm doing the planification of the courses in a university.
I have definned the following tables:

1- Table DAY representing a day (day_id,year,month ...)
2- Table PERIODE representing an hour of a day (periode_id,starting,ending,periode_number)
3- Table TEACHER representing a teacher (teacher_id,name,phone,address ...)
4- Table ROOM representing a ROOM in the university (room_id,description,name ...)
5- Table COURSE representing a course given by a teacher (course_id,name,...)

Now i have to planify the courses:
So i created the
following table PLANIFY(day_id,periode_id,teacher_id,room_id,course_id)

THE PROBLEM IS THAT I DON'T HAVE A WAY TO DEFINE A PRIMARY KEY for the table PLANIFY.
We must note that:
1- a teacher can not give 2 courses in the same peiode on the same day, and can not exist in 2 different rooms in the same periode on the same day.
2- a room can not be occupied by 2 courses or 2 teachers in the same periode on the same day.
3- a course can not be given by 2 teachers or in 2 different rooms

ANY IDEAS ??

SHOULD I CHANGE THE STRUCTURE OF THE TABLE OR DEFINE A TRIGGER (which i don't know how to by the way).

thx for any help.

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