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As a general rule, first class items represent things rather than relationships. For example, the database representations of a human and of a company are each first class items. However, the fact that the person is an employee of that company is not a first class item. Likewise, data
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representing first class items of a given type (e.g. a table of people, a table of companies). It will also contain other tables representing relationships between these first class items.
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independent of any other item. The identity allows the item to persist when its attributes change, and allows other items to claim relationships with the item.
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that relationship, e.g. information about the salary the company pays to its employee, is not a first class item.
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have special identifiers for its rows. Instead these rows will be identified by an ordered
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consisting of unique identifiers of the first class items involved in the relationship.
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between two or more first class items (or data about that relationship) will usually
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assigned to each row (effectively, to each item) as a unique
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