Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Wed, 29 Jun 2005 13:51:47 GMT
Message-ID: <THxwe.14760$eM6.3208_at_newsread3.news.atl.earthlink.net>
"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message news:MPG.1d2c70f48d0be679896bc_at_news.ntnu.no...
> Yes. I know how to draw lines on a paper, but how do you do it on the
> logical level (if there is such a thing)? But it seems you answer that
> further down.
>
Here's some phrasing that I picked up about 20 years ago, when I was learning ER and RDM. I still like it:
"In the ER model, relationships are identified but not implemented."
The idea here is that lines on paper (or elements in a model) specify
relationships, give them names, connect them with entities, and describe
them. But they don't provide a specific data structure for "realizing"
those relationships at retrieval time.
"Foreign keys", implement relationships. They aren't the only possible
implementation, but they are by far the most prevalent in today's practice.