An identifier is a way of uniquely identifying an entity of a particular entity type.
For example, assume that the entity type CUSTOMER has an attribute that is called Number. If no two customers have the same value for their Number attribute, the value of Number can be used to uniquely identify one and only one CUSTOMER entity.
In a consistent model, each entity type must have at least one identifier.
Each entity must be distinguishable from all other entities that are based on some combination of values and pairings.
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