I have a
Parent entity type with two
Child entities, with a relationship cardinality of many-to-one (or
The business rules say that:
- It is mandatory that the
Parententity type be connected to either of
Child Bin any given process, but also that…
Parentcannot be connected to both nor can it be connected to neither during that process.
I am stumped on how to represent the optionality here. It is a mandatory relationship on the
Parent side, but not mandatory in the same way on the
Child side. It is necessary that at least one
Child be present in the relationship at a given time, but I do not feel it is correct to call this mandatory from the
Child side, nor do I feel it is correct to represent both
Child entities in optional relationships with the
Child entity in a mandatory or optional relationship with
This is a theoretical question in my head more-so, but I've ran with the "Who is the parent's favourite child?" example in my diagram as it matches: a child can have many parents favouring them, the parent can have one of either child as their current favourite, and the parent can't have no preference toward one child at a given moment. Cruel, but it works for my example!
I haven't got as far as DB design, though I was assuming that you can set up a column in the Parent table that can accept a Foreign Key from either of
Child_B but has a
Not Null constraint. Or is my assumption off, and as such would this then mean the ERD above can't be implemented like so in the database?