I have a problem in logical design of a SQL Server database.
Still I can not distinct which relation has to be one-to-many and which one has to be many-to-many.
Someone told me if both entity tables are independent, they can have a many-to-many relation, else they will have one-to-many.
But now I am working on a project that collects personal information of the employees, in one part there is a table known as JobStatus which is for the personnel’s current job. This table has a relationship with Person (table) that is many-to-many, of course there is a junction table between them.
I made this type of relation because one job position’s name is assigned to several persons and with different performance.
For instance :
Person A ----->Operator
Person B------>Operator and so on...
And in other side there are some cases that a person has two Job position, I mean he is either a director and a teacher .
For instance :
Person C ------>Director & Teacher
So would you please guide me in this ambiguous logical mean?
Simply put, you would recognize a many to many when either table can not have the PK from the other table as its foreign key
Taking a Student and Course table
Putting a FK of course (CourseID) on student will restrict the Student to ONE course
Putting a FK of student (StudentID) on course will restrict the course to ONE student
To solve this, a third table StudentCourse will have the StudentID and the CourseID, therefore making either table independent.
That is Many to Many.
For one to many, that happens when you can easily put the ID of one table as the FK of the other.
In your case, Two Employees can be Operator at the same time and an Employee can be an Operator and Teacher – that design is MANY to MANY. You are right