Concept

Cardinality (data modeling)

Summary
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. 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 in
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