Re: multi column foreign key for implicitly unique columns

On 8/19/2004 12:52 PM, Oliver Elphick wrote:
> On Thu, 2004-08-19 at 17:21, Josh Berkus wrote:
>> Jan,
>> 
>> > Because the value in b.y is redundant. b.x->a.x->a.y is exactly the same
>> >   value and he even wants to ensure this with the constraint.
>> 
>> And in the absence of that constraint, what ensures that b.y = a.y, exactly?
> 
> In the absence of b.y, it would be impossible for it to be anything
> else.  Isn't that the point?

Precisely. I meant that the entire column is redundant and obsolete. 
Without the column, no need for any constraint.

> It seems to me that he was trying to use the database to show errors in
> his source data, but since his constraint would reject the data, he
> wouldn't be able to enter it; all he could do would be to see the
> error.  So he might as well turn it round, normalise the data properly
> and use the database to tell the rest of the system what the data ought
> to be.

I assumed he often queries b, and to avoid joining a all the time he 
duplicates values from a into b and then tries to ensure that they stay 
in sync with constraints.


Jan

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