Hello i've been reading recently about the ER Model.
The book gave some visual examples about cardinality mapping
From the pictures i can assume that the → means "one" and - means "many"
now in the next chapter, i've encountered something that has been driving me nuts.
by replacing the advisor relationship set with a ternary relationship set, dept advisor, involving entity sets instructor, student, and department that is many-to-one from the pair {student, instructor} to department as shown in Figure 7.6
What! how is that a many to one from {student,instructor} to department ??
how it's not a many to one from {student,department} to instructor.
later he showed a functional dependency that follows the requirement from that ERD
s_ID,dept_name → i_ID
so in that functional dependency s_ID(student_id) and dept_name determines i_ID(instructor_id).
so in other words, we can have many unique combinations of student_id with dept_name to determine instructor_id but the unique combination can determine at most one instructor_id. however the same instructor_id can have many combinations of student_id with dept_name that determines it.
hence it's many to one from {student,department} to instructor
even the functional dependency he showed proves that it's many to one from {student,department} to instructor, i don't understand...