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In computer science, data that has several parts, known as a '' record,'' can be divided into fields (data fields). Relational databases arrange data as sets of
database record In the context of a relational database, a row—also called a 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.rows. Each record consists of several ''fields''; the fields of all records form the
columns 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. ...
. Examples of fields: name, gender, hair colour. In
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of " objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pr ...
, a ''field'' (also called ''data member'' or ''
member variable In object-oriented programming, a member variable (sometimes called a member field) is a variable that is associated with a specific object, and accessible for all its methods (''member functions''). In class-based programming languages, these a ...
'') is a particular piece of
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted. ...
encapsulated within a class or
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
. In the case of a regular field (also called ''
instance variable In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e. a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has simila ...
''), for each instance of the object there is an
instance variable In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e. a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has simila ...
: for example, an Employee class has a Name field and there is one distinct name per employee. A static field (also called '' class variable'') is one variable, which is shared by all instances. Fields are abstracted by
properties Property is the ownership of land, resources, improvements or other tangible objects, or intellectual property. Property may also refer to: Mathematics * Property (mathematics) Philosophy and science * Property (philosophy), in philosophy and ...
, which allow them to be read and written as if they were fields, but these can be translated to
getter A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by . Thus the getter removes small amounts o ...
and
setter The setter is a type of gundog used most often for hunting game such as quail, pheasant, and grouse. In the UK, the four setter breeds, together with the pointers, usually form a subgroup within the gundog group as they share a common fu ...
method calls.


Fixed length

Fields that contain a fixed number of bits are known as fixed length fields. A four byte field for example may contain a 31 bit binary integer plus a sign bit (32 bits in all). A 30 byte name field may contain a person's name typically padded with blanks at the end. The disadvantage of using fixed length fields is that some part of the field may be wasted but space is still required for the maximum length case. Also, where fields are omitted, padding for the missing fields is still required to maintain fixed start positions within a record for instance.


Variable length

A variable length field is not always the same physical size. Such fields are nearly always used for text fields that can be large, or fields that vary greatly in length. For example, a bibliographical database like
PubMed PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institutes of Health maintain the ...
has many small fields such as publication date and author name, but also has abstracts, which vary greatly in length. Reserving a fixed-length field of some length would be inefficient because it would enforce a maximum length on abstracts, and because space would be wasted in most records (particularly if many articles lacked abstracts entirely). Database implementations commonly store varying-length fields in special ways, in order to make all the records of a given type have a uniform small size. Doing so can help performance. On the other hand, data in serialized forms such as stored in typical file systems, transmitted across networks, and so on usually uses quite different performance strategies. The choice depends on factors such as the total size of records, performance characteristics of the storage medium, and the expected patterns of access. Database implementations typically store variable length fields in ways such as *a sequence of characters or bytes, followed by an ''end-marker'' that is prohibited within the string itself. This makes it slower to access later fields in the same record because the later fields are not always at the same physical distance from the start of the record. *a ''pointer'' to data in some other location, such as a
URI Uri may refer to: Places * Canton of Uri, a canton in Switzerland * Úri, a village and commune in Hungary * Uri, Iran, a village in East Azerbaijan Province * Uri, Jammu and Kashmir, a town in India * Uri (island), an island off Malakula Isla ...
, a file offset (and perhaps length), or a key identifying a record in some special place. This typically speeds up processes that do not need the contents of the variable length fields, but slows processes that do. *a ''length prefix'' followed by the specified number of characters or bytes. This avoids searches for an end-marker as in the first method, and avoids the loss of
locality of reference In computer science, locality of reference, also known as the principle of locality, is the tendency of a processor to access the same set of memory locations repetitively over a short period of time. There are two basic types of reference locali ...
as in the second method. On the other hand, it imposes a maximum length: the biggest number that can be represented using the (generally fixed length) prefix. In addition, records still vary in length, and must be traversed in order to reach later fields. If a varying-length field is often empty, additional optimizations come into play.


Example

This Person java class has 3 fields: firstName, lastName, and heightInCentimeters. public class Person


See also

* Class variable *
Data hierarchy Data hierarchy refers to the systematic organization of data, often in a hierarchical form. Data organization involves characters, fields, records, files and so on. This concept is a starting point when trying to see what makes up data and whether d ...
*
Instance variable In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e. a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has simila ...
*
Mutator method In computer science, a mutator method is a method used to control changes to a variable. They are also widely known as setter methods. Often a setter is accompanied by a getter (together also known as accessors), which returns the value of the priv ...


References

{{DEFAULTSORT:Field (Computer Science) Data modeling