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.
For example, consider a database of electronic health records. 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 encounter table with an entry for each hospital visit.
Natural relationships exist between these entities, such as an encounter involving many doctors. There is a many-to-many relationship between records in doctor and records in patient because doctors have many patients and patients can see many doctors. There is a one-to-many relationship between records in patient and records in encounter because patients can have many encounters and each encounter 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.
In data modeling, 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.
A complex data model can involve hundreds of related tables. Computer scientist Edgar F. Codd created a systematic method to decompose and organize relational databases.
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thumb|upright=1.2|Structure générale d'une base de données relationnelle. En informatique, une base de données relationnelle (en anglais: relational database management system (RDBMS)) est une base de données où l'information est organisée dans des tableaux à deux dimensions appelés des relations ou tables, selon le modèle introduit par Edgar F. Codd en 1960. Selon ce modèle relationnel, une base de données consiste en une ou plusieurs relations. Les lignes de ces relations sont appelées des nuplets ou enregistrements.
La cardinalité ou multiplicité, dans les schémas relationnels en modélisation des données, sert à compter le nombre minimum et maximum de possibilités que chaque classe contient dans la relation liant deux ou plusieurs objets. Cette notion est utilisée par la modélisation Merise et UML. Avec UML par exemple, si une classe Client peut avoir de 0 à 2 adresses, on parle d'une multiplicité 0..2. La multiplicité est la définition du nombre possible de relations entre les Types alors que la cardinalité définit le nombre de relations réellement paramétrées entre les Objets issus de ces Types.
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