database-designerd
I'm creating an ER diagram for a class assignment and I was wondering if it would be possible to create an entity that had no direct relationship to any other entity, and simply use foreign keys to connect them. The diagram below illustrates what I'd like to do
Basically, my diagram is starting to get really complex and it would be great if I didn't have to maintain a connection between every single entity.
FWIW, I'm using Chen and Crow's Foot notation, if that's any help.
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.).
Your first option works best when there is the possibility that a lecture can have multiple tutors or when a tutor can teach multiple classes. Your second option works best when you can guarantee that a class will only ever have a single tutor at any one time (otherwise you would need multiple entries in your class table to accommodate extra tutors). The third one will only work if you can guarantee that a tutor will only ever teach one class (the opposite of the second scenario).
If you want/need maximum flexibility then go with option 1. However, you should choose the option that best suits your needs based on the descriptions of the types I have given above.
You can indeed over-normalize a database, and in some cases a de-normalized structure may suit the requirement better - but I don't think this is one of those times.
I hope this helps you.
Best Answer
An ER diagram is merely a tool to aid you in development. An ER diagram doesn't even by mandate require you to show all the entities in the system.
So to answer your question by one route: It's perfectly acceptable to have an entity on a diagram that has no relationship to any other entity. Consider a settings table that is used by a simple application to store the settings for the program. It would not need any other entities to do its job. But it would still be an object which existed within the scope of the datastore.
So to answer your question by another route: You can display as many or as few diagrams on your ER diagram as are required to show the information you're trying to represent. But if you choose to omit a relationship for purposes of clarity, it would still make sense to make a note that the R is omitted. I would personally but a
-{#}with a number inside the related circles for each entity, with a footnote for that # at the bottom of the page. That way other readers won't have to ask if there should be another relation.That's my $0.05 worth on this one anyways. The point here is that this is partly a matter of style. Some people will tell you that you always have to put the relationships (like, for instance, your boss might require that, and that would trump my advice). But the final truth is what's in the database.