First class (computing)
Encyclopedia
In database model
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
will include several table
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
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.
Database model
A database model is the theoretical foundation of a database and fundamentally determines in which manner data can be stored, organized, and manipulated in a database system. It thereby defines the infrastructure offered by a particular database system...
ing, a first class item is one that has an identity
Identity (object-oriented programming)
An identity in object-oriented programming, object-oriented design and object-oriented analysis describes the property of objects that distinguishes them from other objects. This is closely related to the philosophical concept of 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
Relational database
A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...
will include several table
Table (information)
A table is a means of arranging data in rows and columns.Production % of goalNorth 4087102%South 4093110% The use of tables is pervasive throughout all communication, research and data analysis. Tables appear in print media, handwritten notes, computer software, architectural...
s, each of which contains rows
Row (database)
In the context of a relational database, a row—also called a record or tuple—represents a single, implicitly structured data item in a table. In simple terms, a database table can be thought of as consisting of rows and columns or fields...
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
Column
A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...
of the table will typically contain a different integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
assigned to each row (effectively, to each item) as a unique identifier
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
: 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
Tuple
In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...
consisting of unique identifiers of the first class items involved in the relationship.