Once a 1:1 relationship has been identified, the entity that receives the key attribute from its associated entity may be determined by identifying the type of participation each entity has in the relationship. (Types of participation are discussed later in this module.)
If identifying the type of participation for each entity fails to help (and it often fails to help), the decision is arbitrary as to which entity in the relationship receives the key attribute to form the link.
Before tackling a 1:1 relationship, we need to know its optionality.
There are three possibilities the relationship can be:
- mandatory at both ends
- mandatory at one end and optional at the other
- optional at both ends