Re: Entity Relationship Diagrams

From: Roy Hann <specially_at_processed.almost.meat>
Date: Wed, 11 Apr 2007 10:45:50 +0100
Message-ID: <7eOdnaszLrvSM4HbnZ2dnUVZ8tGqnZ2d_at_pipex.net>


"Gregc." <gregchilton_at_bigpond.com> wrote in message news:1176284039.467335.142540_at_b75g2000hsg.googlegroups.com...
> Hi
>
> I am currently drawing and ER diagram and am not sure about how to
> draw the diagram for the following: A bike must have at least 2 wheels
> but it may have more. The way that I would draw it is thus:
>
> BIKE-------< WHEELS (ie crows feet with no line)
>
> Would anyone have a suggestion on this.

ER diagrams quickly run out of steam when you try to represent arbitrary constraints, so don't expect much from them. Admittedly this is a trivial problem that ought to be within the power of any ER diagram notation, but you'd have to specify which of the very many such notations you are using.

Personally, if I were to bother with an ER diagram at all I would use a notation in which two bars represent your rule, rather than a "two toed" crows foot, but there is at least one commonly used notation that says two bars actually means "one-and-only-one". Nuts. But ER diagrams don't float my boat sufficiently to complain about it to anyone (before now).

Roy Received on Wed Apr 11 2007 - 11:45:50 CEST

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