Am 18.05.2013 um 13:24 schrieb andy pugh <bodge...@gmail.com>:
> On 18 May 2013 06:15, Michael Haberler <mai...@mah.priv.at> wrote:
>
>> tools(toolid, length, diameter, other-non-offset parameters)
>> offsets(offsetid, xoffset,yoffset..., purpose) (purpose could be:
>> coordinate system offset, wear, tool holder..)
>>
>> then you have a relation describing the active offsets, which is 1:N in
>> nature (a tool can have an arbitrary number of offsets or none at all):
>>
>> tooloffsets(toolid, offsetid) (only toolid is a primary key)
>
> I think toolid is the wrong primary key there, assuming that primary
> keys are unique. I think that the "relation" (if I have my technical
> terms correct) is
>
> tooloffsets(tooloffsetid, toolid, offsetid)
>
you are right, good find - what needs to be unique is the tuple {toolid,
offsetid}
> To take a concrete example, tools 11 and 22 both with offsets 33 and
> 44 applied would be:
> tooloffsetid toolid offsetid
> 1 11 33
> 2 11 44
> 3 22 33
> 4 22 44
the usual way to handle this is to define the tuple {toolid, offsetid} as
primary key, which will avoid duplicates of that tuple (that could happen in
theory if the only primary key is tooloffsetid like so:)
> 1 11 33
> 2 11 33
>
> --
> atp
> If you can't fix it, you don't own it.
> http://www.ifixit.com/Manifesto
>
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