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Within data modelling, cardinality is the numerical relationship between rows of one table and rows in another. Common cardinalities include ''one-to-one'', ''one-to-many'', and ''many-to-many''. Cardinality can be used to define data models as well as analyze entities within datasets.


Relationships

For example, consider a database of
electronic health records An electronic health record (EHR) is the systematized collection of electronically stored patient and population health information in a digital format. These records can be shared across different health care settings. Records are shared thro ...
. Such a database could contain tables like the following: * A doctor table with information about physicians. * A patient table for medical subjects undergoing treatment. * An appointment table with an entry for each hospital visit. Natural relationships exist between these entities: * A many-to-many relationship between records in doctor and records in patient because doctors have many patients and patients can see many doctors. * A one-to-many relationship between records in patient and records in appointment because patients can have many appointments and each appointment involves only one patient. * A one-to-one relationship is mostly used to split a table in two in order to provide information concisely and make it more understandable. In the hospital example, such a relationship could be used to keep apart doctors' own unique professional information from administrative details.


Modeling

In
data modeling Data modeling in software engineering is the process of creating a data model for an information system by applying certain formal techniques. It may be applied as part of broader Model-driven engineering (MDE) concept. Overview Data modeli ...
, collections of data elements are grouped into "data tables" which contain groups of data field names called "database attributes". Tables are linked by "key fields". A "primary key" assigns a field to its "special order table". For example, the "Doctor Last Name" field might be assigned as a primary key of the Doctor table with all people having same last name organized alphabetically according to the first three letters of their first name. A table can also have a foreign key which indicates that field is linked to the primary key of another table.


Types of models

A complex data model can involve hundreds of related tables. Computer scientist
Edgar F. Codd Edgar Frank "Ted" Codd (19 August 1923 – 18 April 2003) was a British computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases and relational database ...
created a systematic method to decompose and organize relational databases. Codd's steps for organizing database tables and their keys is called
database normalization Database normalization is the process of structuring a relational database in accordance with a series of so-called '' normal forms'' in order to reduce data redundancy and improve data integrity. It was first proposed by British computer scien ...
, which avoids certain hidden database design errors (delete anomalies or update anomalies). In real life the process of database normalization ends up breaking tables into a larger number of smaller tables. In the real world, data modeling is critical because as the data grows voluminous, tables linked by keys must be used to speed up programmed retrieval of data. If a data model is poorly crafted, even a computer applications system with just a million records will give the end-users unacceptable response time delays. For this reason, data modeling is a keystone in the skills needed by a modern software developer.


Database modeling techniques

The
entity–relationship model An entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can e ...
proposes a technique that produces entity–relationship diagrams (ERDs), which can be employed to capture information about data model entity types, relationships and cardinality. A Crow's foot shows a one-to-many relationship. Alternatively a single line represents a one-to-one relationship.


Application program modeling approaches

In the object-oriented application programming paradigm, which is related to database structure design, UML class diagrams may be used for
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 a ...
modeling. In that case, object relationships are modeled using UML associations, and multiplicity is used on those associations to denote cardinality. Here are some examples:


See also

*
Arity In logic, mathematics, and computer science, arity () is the number of arguments or operands taken by a function, operation or relation. In mathematics, arity may also be called rank, but this word can have many other meanings. In logic and ...
* Entity-relationship model *
Unified modeling language The Unified Modeling Language (UML) is a general-purpose visual modeling language that is intended to provide a standard way to visualize the design of a system. UML provides a standard notation for many types of diagrams which can be roughly ...


References


External links


UML multiplicity as data model cardinality
- http://www.agiledata.org
Cardinality in Data Modeling
- Adam Alalouf, Temple University
Cardinality
on Techopedia
Cardinality
on Geeksforgeeks
Database Cardinality
on SQL World {{DEFAULTSORT:Cardinality (Data Modeling) Data modeling Relational model de:Kardinalität (Datenbanken) pt:Cardinalidade (modelagem de dados)