The term identifier is also used for primary key. The primary key selection must follow certain criteria. Key should not change its value over the life of each entity instance. Substitute single-attribute surrogate keys for large composite keys. A relationship is an association between instances of one or more entity types.
Most relationships are binary.
This type of relationship involves two entities. We say the relationship degree is equal to 2 because two entities participate in the relationship. Examples include the following shown in Figure 3. Note how intersection attributes are modeled. This represents an association between occurrences of a single entity.