Ε-R diagram with conditions , is it possible? I want to create a database and model it with an E-R diagram. This database should be about a library. An entity is the member of the library, where the member can borrow books from the library. However , the member cannot borrow more than 5 books and cannot borrow any more books , if he hasn't returned every book of which the time to return it has expired. Ιs it possible to model this condition with an E-R diagram?
This can be expressed in a one to many relationship with a 0..5 end:

LibraryPatron 1 &lt;----> 0..5 BorrowedBook  

Using relational algebra they can be expressed using check constraints as Renzo suggested.   Some ER tools may also allow you to express this as a constraint.