Is the following scenario correct and why:
- A and B are two entities
- B is-a A
- A is-a B
And what if A, B were relationship?
Entity-Relationship Diagram – Representing Mutual IS-A Relationship
database-designerdsubtypes
Related Solutions
The key to understand this case is at the ninth page from Doc-A:
"product" and "customer" are entity types, and "shipped to" is a relationship... the verb "ship" is then converted to a gerund... in other words, the relationship... has been converted into the entity type "shipping"
Hence, the final third form will have "shipped to" converted to an entity by itself, "shipping".
This is necessary because "Shipped to" is a MANY-TO-MANY relationship (although to myself Chen's text is not clear on this).
But, your example does not relate - the relationship "Identified by" is an ONE-TO-MANY (a card belongs to ONE customer / a customer have ONE OR MORE cards issued).
Hence, the model would have the relationship between "Bank" and "Bank card" identified by TWO verbs.
Or, making the model stronger, to register several authentication events, one may turn "Authenticated by" into a entity by itself, "Authenticating" (to register event date, data shared, etc.).
I think you can make an Orders entity that is a relationship between Products and Customers. The Orders would have subclasses FoodOrders and ShoeOrders, which respectively would be the relationships between the subclasses of Food - GroceryStores and Shoes - ShoeStores.
┌───────────┐
│ Employees │
└─────┬─────┘
│
│
┌──────────┐ ┌────────┐ │ ┌───────────┐
│ Products ├─── contain ───┤ Orders ├─── deliver ───┤ Customers │
└────┬─────┘ └────┬───┘ └─────┬─────┘
│ │ │
│ │ │
isA isA isA
│ │ │ │ │ │
┌────┘ └────┐ ┌─────┘ └─────┐ ┌────┘ └────┐
│ │ │ │ │ │
┌───┴──┐ ┌───┴───┐ │ │ ┌───────┴───────┐ │
│ Food │ │ Shoes │ │ │ │ GroceryStores │ │
└───┬──┘ └───┬───┘ │ │ └───────┬───────┘ │
│ │ ┌─────┴──────┐ │ │ ┌─────┴──────┐
└───────────│───────┤ FoodOrders ├──────│────────────┘ │ ShoeStores │
│ └────────────┘ │ └─────┬──────┘
│ ┌─────┴──────┐ │
└─────────────────────┤ ShoeOrders ├─────────────────┘
└────────────┘
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Best Answer
In databases, subtyping means that one domain is a subset of another. If A is a subset of B and B is a subset of A, then it means A = B. The same logic applies whether A and B are entity sets or relationship sets. There would be no point distinguishing A and B in an ER model.