Editing First class (computing)

{{For|general usage outside databases|First-class object}}
{{Unreferenced|date=December 2009}}

In [[database model]]ing, a '''first class''' item is one that has an [[identity (object-oriented programming)|identity]] 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.

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 ''about'' that relationship, e.g. information about the salary the company pays to its employee, is not a first class item.

Typically, a [[relational database]] will include several [[table (information)|table]]s, each of which contains [[row (database)|rows]] 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. 

In a table representing first class items, one [[column]] of the table will typically contain a different [[integer]] assigned to each row (effectively, to each item) as a unique [[identifier]]: that is to say, unique for objects of this type; objects of different types, represented in different tables, can coincidentally have the same identifier, but the coincidence is meaningless.

In a relational database, a table representing a ''relationship'' between two or more first class items (or data about that relationship) will usually ''not'' have special identifiers for its rows. Instead these rows will be identified by an ordered [[tuple]] consisting of unique identifiers of the first class items involved in the relationship.

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[[Category:Data modeling]]