Is this Library Management System ER diagram correct?

Question

Quick question about an ER/EER Diagram.

I have made this Entity Relationship Diagram, but I have been told, that there is something wrong with it by a friend. Is there something wrong with it?

The ER diagram is a design of a Library Management System, where a member can borrow 5 books at a time. The rest of the functionality of the system is how a normal library functions.

Library Management System EER

Answer 1

i don't understand the utility of the relationship between the librarian and the card and i don't understand why the books are splitted in two entities.

I would do 3 entities:

-member

-card

-book

every member has one card, every card is of one member; every member can take many books, every book can be taken by many members,

the relation between member and book create another table in the logic schema: loans. before inserting a new loan you can check if the member has alredy 5 active loans (by checking the attribute active in the loans table).

Answer 2

Your given context is incomplete for me. I do not see the whole description of your problem/situation, so I will answer based on assumptions, and the experience I had during my life. So let's see...

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The tino user questioned the existence of two entities, title and volume, which is something important. Let me explain this for a moment, which will eliminate this as an error. Previously (a time ago) we had video rental stores (I don't know if this the right name where you live, english is not my native language). Remember? We used to go there to rent VHS tapes to watch at home.

What we rented were not films, but more copies/midia of them. A film will always have the same actor, director, title, etc., but a copy could have different attributes/properties, like the year that the media was manufactured, the available languages, the expiration year, among other things. So we had distinctly two different things.

But despite this, we have to consider whether there is a need to create two entities for persistence. We have to remember if we need to persist this information. If a copy/midia has no attributes, then it's entity should not exist, and what a user would rent really would be the movies titles.

In your case, the relationship between volume and title, I belive, is really expressing this discrepancy.

Let's talk about the relationship between librarian and title. What a librarian manages? Does It manages the titles that never change and are abstract things, or the physical objects present in the library? :)

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Finally, let's talk about the borrows relationship. When we break down 1-N (or N-1) relationships, we always pass the primary key from the 1 side to the N side, solving the relationship to the formation of the Physical Model in a Entity-Relationship Diagram.

Despite this relationship here is a 0-5, to decompose it, we will not have exactly a 0-5 relationship. We would have in anyway to pass the primary key from the two sides to the table formed by this relationship. Therefore, here we have initially a NN relationship between member and volume.

NN relationships allow optional relations between entities. This means we can have the zero side cardinality here. To limit the number of books that can be rented, you need to implement a restriction/constraint with SQL, or with any procedural language in your database. In this case, you can implement a before insert trigger. This trigger has a duty to verify this restriction to allow or denny the completion of the operation as a whole.

Let it be clear that I'm not saying you should remove this notation. Your Conceptual Model should express it. But when you are decomposing, you have to remember that. I think you should just correct it.

Remember one important rule: Relations that have attributes/properties (the attributes/properties) can only exist in NN relationships. If you have to put attributes/properties in a 1-N (or a N-1) relation, they (the attributes/properties) will always be on the N side. In summary, there are no N-1 (or 1-N) relationships with attributes in the relation. Only NN relations can have attributes/properties. So be careful with this.

Any questions or clarification, please comment and I will answer.

Answer 3

I see no reason to distinguish member and card. Volume and Librarian don't have primary keys. Are they supposed to be weak entities? That doesn't make sense for Librarian and Volume needs an identifier to distinguish different copies.