Re: ER Diagramming Question

On Mar 26, 11:27 am, "Michael P." <mich..._at_michaeljpatterson.com> wrote:
> This may not be the most appropriate group to post this question, but
> any number of the users will probably have the answer.
>
> When reading an ER diagram using Crow's feet notation, it's my
> understanding that a single has mark means that end of the
> relationship is mandatory. Does a double has mark mean anythign
> different or is that how the ERD tool represents that relationship?
>
> Also does the optional symbol on top of a has have any extra meaning.
> I've run into this on the many side of the relationship?

Michael,
The single indicates one and the crow's feet indicates many.

Optionality would be indicated by solid (must) and dashed (may have) notation.

The classic dept-to-emp relationship, there would be a single, dashed line on the dept end and crow's feet, solid line on the emp end, indicating a dept may have one or more emps and an emp can must have only one dept. (This model is a little dated. Afterall, what emp stays in only one dept for the duration of their tenure, but I digress...)

The above is true for Oracle's Designer ERD modeling tool. Not sure what other products use.

Vince Received on Mon Mar 26 2007 - 15:17:13 CDT