Exercise $2.$ Consider the following relations:

Students$($sid: string, name: string, login: string,

age: integer, gpx real$)$

Faculty$($fid: string, fname: string, sal: real$)$

Courses$($cid: string, cname: string, credits: integer$)$

Rooms$($mo: integer, address: string, capacity; integer$)$

Enrolled$($sid: string, cid: string, grade: string$)$

Teaches$($fid: string, cid: string$)$

Meets$\_$In$($cid: string, mo: integer, time: string$)$

a$)$ List all the foreign key constraints among these relations.

b$)$ Give an example of a $($plausible$)$ constraint involving one or more of these relations that is not a primary

key or foreign key constraint.