Use of Entity or Relationship Sets

It is not always clear whether an object is best represented by an entity set or a relationship set.

• Both Figure 2.13 and Figure 2.22 show account as an entity.
• Figure 2.23 shows how we might model an account as a relationship between a customer and a branch.

    
  Figure 2.23: E-R diagram with account as a relationship set
  

• This new representation cannot model adequately the situation where customers may have joint accounts. (Why not?)

• If every account is held by only one customer, this method works.