Avoid Circular Dependency

Question

I am developing a travel management application. The design in question is something like following :

Each person in a tour is designated as a Traveler. Each Traveler ha

Answer 1

As you can see, the three tables - Traveler, MainMember and SubMember - have formed a circular dependency.

No, this is not a circular dependency - no "node" in this graph can reach itself by following graph "edges" in their proper direction¹. It would be more accurately described as "merged" or even (proto) "diamond-shaped" dependency.

And no, it will not hurt your application.²

You are effectively implementing inheritance (aka category, subclassing, subtyping, generalization hierarchy etc.), using "each class in separate table" approach. This approach is clean, but cannot guarantee presence³ and exclusivity⁴ out of box. There are ways to amend it so it does, and there are other implementation strategies for inheritance that can, but they have their own pros and cons.

In my opinion, your model is just fine for what you are trying to accomplish and enforcing the presence/exclusivity at the application level is probably lesser evil than suffering the cons of trying to enforce it at the database level.

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¹ "Direction" is from referencing to referenced table. Traveller doesn't reference MainMember nor SubMember and therefore breaks the cycle.

² Unless you are on a DBMS (such as MS SQL Server) that doesn't support referential actions on such "merged" dependencies (in which case you'd need to implement ON CASCADE DELETE through triggers).

³ Traveller cannot exist alone (cannot be "abstract"), it must be MainMember or SubMember.

⁴ Traveller cannot be both MainMember and SubMember.