Re: Newbie question on cardinality in a data model

You are confusing "data model" with "relational schema". A data model can be expressed in a drawing known as an ERD (Entity Relationship Diagram). This is an abstract view of the data that is being modeled and the realtionships among the data. The data is modeled as entities and relationships among the entities. Cardinality is an important part of this model, as well as are participation constraints (mandatory/optional). Once this ERD is *properly* constructed, it is possible to convert it into a relational schema, which is the expression of how your tables will be layed out (sometimes called a "physical" model, though I find that a confusing term). Many people confuse the entities in an ERD with tables. They are not tables. Some may become tables, but others will combine with relationships and other enities to become tables in the "physical" model (relational scherma). Cardinality constraints help determine how the entities and relationships get combined into tables. There is actually an algorithm to follow to do this, though many people (even on these newsgroups) don't know about it.

I am not familiar with Cognos Reportnet, but I am familiar with some of their other reporting tools (Impromptu, PowerPlay). I suspect Reportnet allows you to create reports from an ERD "view" of your data. End users are often comfortable with (or at least understand) ERDs, so it may be that this tool allows an end user to create reports from a diagram he/she may understand, rather than from a relational schema, which may be harder to understand.

"Anna" <post_at_pallina.ch> wrote in message news:547b1c8.0310311130.6e68d3a4_at_posting.google.com...
> Hi,
>
> could somebody please explain why it is so important in a data model
> to specify the cardinality of a relationship?
> From what I understand, if I query a database via SQL the statement I
> write does not depend on the cardinality (with the exception of outer
> joins) of a relationship. Yet my reporting tool (Cognos Reportnet)
> requires me to set the cardinality of each relationship. Why is it
> necessary, and what happens if I set it wrong? Any help or reference
> to reading material is greatly appreciated!
>
> Thanks,
>
> Anna
Received on Fri Oct 31 2003 - 14:15:55 CST