Talk:Cardinality (data modeling)

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Can't a doctor have multiple patients and a patient multiple doctors? Wouldn't the example mentioned then be of a many-to-many and not many-to-one type?

Well it seems to be correct there: there is a many-to-many example regarding doctors and patients and one-to-many between doctors and departments. Zeratul021 (talk) 21:12, 15 January 2009 (UTC)[reply]

In this article the relational model is mixed up with ER-Modeling! There is no n:m relationship between relational tables. In ER modeling the cardinality of a relationship type is a constraint that limits the number of potentially related entities.

In the relationsal data model the cardinality of a table is the number of tuples (rows) in that table. —Preceding unsigned comment added by 84.147.211.226 (talk) 21:02, 7 May 2010 (UTC)[reply]

This article is crap. Firstly, the previous commenter is correct, the cardinality of a table is the number of distinguishable rows in that table. However, a far more common use of the term in datamodelling is in respect of a column or combination of columns. In this context the term refers to the degree to which a column value differentiates its row from other rows: a column with "high cardinality" is a good candidate for indexing. Secondly, Date did not invent a method, he devised a model. There are endless ways that model can be applied, and while Date certainly wrote a seminal text on the application of his model, it is absurd to conflate the model with possible ways to apply it, not to mention irrelevant to the meaning of cardinality in datamodelling. Thirdly, normalisation serves to prevent update anomalies and eliminate redundant storage. It does not improve performance in and of itself, although it will often afford opportunities for better indexing. — Preceding unsigned comment added by PeterWone (talk • contribs) 05:59, 21 December 2011 (UTC)[reply]