Re: Identifying an Identifying Relationship

Thank you. Helps clarify it much more than the websites I found on this.

"Jan Hidders" <hidders_at_REMOVE.THIS.win.tue.nl> wrote in message news:9mjg11$7a7$1_at_news.tue.nl...
> Ok. But don't forget that next to the surrogate key there should at
> least be one other real key.

Yes, there are 'real-world' keys that users use to lookup information. The surrogate keys are never shown to the user, but used internally to maintain structure and integrity. The real-world keys are often unique, meeting one of a primary key's requirements, but are rarely immutable. In the extremely rare case where indeed they cannot change, we use a surrogate key anyway for consistency.
>
> That is actually not a very good example. A relationship between A and
> B is only an identifying relationship if two (!) things hold:
> 1. Bs cannot exist without an associated A
> 2. Bs can only be identified through their relationship with As
>

This definition really clears it up for me. Is there a good source for this kind of information. My copy of SQL for Smarties, and my way out of date Date books (1977) don't mention identifying relationships.

This leads to a personal nitpick for me. It would seem to me that the number of relationships that meet both 1 and 2 above is somewhat limited. IMHO, most relationships would be non-identifying. That being the case, why are the rare identifying relationships drawn with a solid line, (easy to draw and for the eye to follow) and the common non-identifying relationships drawn with a dashed line (more difficult to draw and follow with the eye). When I use ERWIN to model the database for management, they wonder why I keep using dashes. And since apparently there is no way in ERWIN to prevent it from appending keys, I live with and explain the ugly lines every time somebody new looks at the chart. Like I said, a nitpick.

Thank you for the response!

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