In
database model
A database model is a type of data model that determines the logical structure of a database. It fundamentally determines in which manner data can be stored, organized and manipulated. The most popular example of a database model is the relatio ...
ing, a first class item is one that has an
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
A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970.
A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured for ...
will include several
table
Table may refer to:
* Table (database), how the table data arrangement is used within the databases
* Table (furniture), a piece of furniture with a flat surface and one or more legs
* Table (information), a data arrangement with rows and column ...
s, each of which contains
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
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member ...
of the table will typically contain a different
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
assigned to each row (effectively, to each item) as a unique
identifier
An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, person, physical countable object (or class thereof), or physical mass ...
: 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
In mathematics, a tuple is a finite sequence or ''ordered list'' of numbers or, more generally, mathematical objects, which are called the ''elements'' of the tuple. An -tuple is a tuple of elements, where is a non-negative integer. There is o ...
consisting of unique identifiers of the first class items involved in the relationship.
{{DEFAULTSORT:First Class (Computing)
Data modeling