Fig. 3.15 Deriving a 1:1 (one:one) relationship.
Question 1: How many birth certificates has a person? Answer: One.
Question 2: How many persons is a birth certificate owned by? Answer: One.
Where there is a one-one relationship type we have the option of merging the two entity types. The birth certificate attributes may be considered as attributes of the person and placed in the person entity type. The birth certificate entity type would then be removed. There are two reasons for not doing this. Firstly, the majority of processing involving PERSON records might not involve any or many of the BIRTH_CERTIFICATE attributes. The BIRTH CERTIFICATE attributes might only be subject to very specific processes which are rarely executed. The second reason for not merging might be that the BIRTH CERTIFICATE entity type has relationship types to other entity types that the PERSON entity type does not have. The two entity types have different relationship types to other entity types.