Orphans

Question

Hi,
Foreign key null may result in orphaned records.
As a null may have no parent. why is this allowed?
What conditions require this?
Regards.
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Answer 1

There are scenarios which would require this kind of setup
1. Consider case of self referencing relationship as in case of employee and his manager. Typically this would be represented by a single table having fields EmpID and MgrID which will point to EmpID of same table itself via foreign key. So in this case
you may choose to make foreign key column MgrID nullable type to represent root level employee (CEO) who will not have a manager to report. If you want to enforce NOT NULL here then you might have to define a value to represent absence of manager like -1/0
which would require a record with id as same value present in table to denote absence of manager condition.
2. Consider case where an entity may belong to more than one group. take case of School Library. Here we will have Member table which will indicate member of library. The member can be a student, teacher or even a non teaching staff within school. So the
table will have separate columns to indicate StudentID,StaffID,TeacherID (or if you want you can collapse teacher and other staff info into one main table) and each column will be linked to master table by means of foreign key constraint (so StudentID ->
Students Table id field,StaffID -> Staff table id etc). So if you take any one record it will have value for only one of fields ie the member is either a student or teacher or non teaching staff. This is a case where you make them nullable.
So scenarios like above you'll always make FK column as of type NULLable
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