Hello, a very good eveing to you all.
I have a question regarding er modelling that has been plaguing me for a few days now.
I will have to develop an eer from my er and the simple problem is as follows.
Suppose i have 2 entities University and School which are stated explicitly.
Now suppose a University provides a studentlibrary (entity) with attributes staff books etc...
And suppose School provides a publicLibrary (entity) with identical attributes as the above (i.e staff books etc..) and both have same primary key

whats the best way to represent this in an er model.I have just [university] (relationshiprovides) [studentlibrary] and also [school] (relationshiprovides) [publiclibrary].this seems a bit redundant though.Should i just have 1 entity called [ibrary]instead of the [studentlibrary], [publiclibrary] entities.if i do this how will it play in my eer model,
Please help,
thanking you all.
regards confused

damn it, should be (relationshipprovides)) instead of those smily faces, sorry

for gods sake, (realtionship "provides") there that should do it.not use to these smilies, ive disabled them now!

use two entities, [studentlibrary] and also [publiclibrary]

trying to combine them will lead to headaches

when the time comes to implement them in a physical database, there are all kinds of tricks that you can use in order to implement only one library table (e.g. mutually exclusive optional foreign keys to two different parent tables), but for conceptual and logical modelling, you definitely want to keep them separate

gotcha.makes sense!just to clarify even though they have the exact same attributes and primary key, ill keep them seperate.Will they still be seperate in the EER model too?
Finally for what reason would it cause headaches combining them (just curious)!
thanks a million for your help!

what is EER? is that like ER?

headaches? well, just try having the same entity be the mutually exclusive child of two parents

i have no idea how you would draw that

This is how you would do it using the Barker ERD notation, which is popular with Oracle people like me:
(That's the best I could do graphically: my attemps to upload a bitmap always fail!)

The kind of bracket-thing joining the two relationships is called an "arc", and it means in this case: "a Library must be associated with EITHER a University OR a School (not both)".

However, an alternative would be to use subtypes:

hi thanks for both of your help.Just to briefly inform people reading this topic, EER modelling means "Enhanced Entity Relationship" Modelling and enhances an ER model to include generalization/specialization, inheritance, disjoint/non-disjoint relationships between sub and super classes and participation constraints etc...

In my eer model i now believe i will have a Library superclass that has entities (books, staff, members...) and it will have 2 subclasses universityLibrary and SchoolLibrary within a disjoint relationship, meaning that a library can be a university library "OR" a school library but never both because they are seperate in their own contexts even though they have the same entities.

One final question
within my "ER" diagram (as r937 commented) should i have something like:


|university|---------"provides"-------|studentlibrary|

"cardinality 1:1"



|school |---------"provides"--------|schoollibrary|

"cardinality 1:1"

and then have superclass (library) and subclasses (studentlibrary and universitylibrary entities) in my eer diagram as mentioned the part about my EER diagram and similar to what Mr. Andrews posted?
again many thanks for your informative help and contribution,
regards!