Examples: an employee entity might have a Social Security Number (SSN) attribute, while a proved relationship may have a date attribute. All entities except weak entities must have a minimal set of uniquely identifying attributes which may be used as a unique/primary key. Entity-relationship diagrams (ERDs) don't show single entities or single instances of relations. Rather, they show entity sets (all entities of the same entity type) and relationship sets (all relationships of the same relationship type). Examples: a particular song is an entity; the collection of all songs in a database is an entity set; the eaten relationship between a child and his lunch is a single relationship; the set of all such child-lunch relationships in a database is a relationship set. In other words, a relationship set corresponds to a relation in mathematics, while a relationship corresponds to a member of the relation. Certain cardinality constraints on relationship sets may be indicated as well. Mapping natural language Chen proposed the following guiding rules for mapping natural language descriptions into ER diagrams: "English, Chinese and ER diagrams" by Peter Chen. Physical view show how data is actually stored. Relationships, roles and cardinalities In Chen's original paper he gives an example of a relationship and its roles. He describes a relationship "marriage" and its two roles "husband" and "wife". A person plays the role of husband in a marriage (relationship) and another person plays the role of wife in the (same) marriage. These words are nouns. That is no surprise; naming things requires a noun. Chen's terminology has also been applied to earlier ideas. The lines, arrows and crow's-feet of some diagrams owes more to the earlier Bachman diagrams than to Chen's relationship diagrams. Another common extension to Chen's model is to "name" relationships and roles as verbs or phrases. Role naming It has also become prevalent to name roles with phrases such as is the owner of and is owned by. Correct nouns in this case are owner and possession. Thus person plays the role of owner and car plays the role of possession rather than person plays the role of, is the owner of, etc. The use of nouns has direct benefit when generating physical implementations from semantic models. When a person has two relationships with car then it is possible to generate names such as owner_person and driver_person, which are immediately meaningful. Cardinalities Modifications to the original