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Re: [ProgSoc] NULL values in primary keys in MySQL



On Sat, Jun 03, 2006 at 11:54:48PM +1000, jedd wrote:
>  I understand it's bad design, and I get that what I probably want
>  is a unique key rather than a primary one, and I get that I should
>  read up (again) on normalisation, and I'm well aware that NULL
>  != NULL  ... but all that aside (and AFAIK that all *is* an aside)
>  what's the logical, or even technical reason for why you can't have
>  a primary key that contains columns that may contain a NULL entry?

Because a primary key requires all keys to be of a unique value and a
NULL does not mean anything other then (effectively) unset, unknown or
words to similar effect. In essence it is not actually a value and as
such it can't be compared to a value in order to test for uniqueness.

arp

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