Entity Relationship diagram

     
      Fig: E-R diagram corresponding to customers and loans

Fig shows an example of an entity-relationship diagram. Such a diagram must have the following criteria:

·         Entities represented by rectangles (e.g. customer and loan)

·        Attributes of an entity represented by ellipses (e.g. loan_number and amount attributes for the entity loan)

·        Relationship sets represented by diamonds (e.g. borrows relationship set.)

·        Primary key attribute of an entity set underlined (e.g. primary key customer_id underlined for entity customer)

·        Links joining diamond relationship set to entities. (e.g. there are links joining the entities to the borrows relationship to complete the E-R diagram.)

·        Fig depicts a binary relationship between the two entities customer and loan. The cardinality ratio from customer to loan is one-to-many because a customer can borrow many loans from the bank but one loan from the bank can belong to one customer assuming the loan is not joint i.e, an individual loan.