If there are mapping constraints in the E-R diagram , can we reduce the size of the table ?

Question

I am not getting that how many attributes must be there in the Borrowed by table which actually represents a table since we have the constraint that one user can borrow only one book .

Comments

We need 

UID, BID, DOI 

What is the confusion here?

Even if one user can take more books, this won't change. 

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Sir , actually I had just this confusion that since I am going to take only the key attributes so then I guess this would always be the criteria for defining the relationship table , I was confused if we could reduce the size of the table but since we need key attributes hence we can't , now I understood , thankss.

Answer 1

in above given e-r diagram one is to one relation is given since it hav both two arrows facing towards entity it means that one user can take only one user and also conversly says that one book is borrowed by one user so in both entity book and user one primary key that must be present in borrowed table i.e. accesetion no and  card no. and one attribute is DOI of borrowed relation so minimum attribute will be 3