Re: Need help with 'unique parents' constraint

Leif B. Kristensen wrote:
> On Sunday 11 September 2005 16:04, Greg Sabino Mullane wrote:
>
>
>>Not just old-fashioned, it's the biological law! (among homo sapiens
>>anyway). I'd approach this with a trigger, as you can do complex
>>checks and get back nice customized error messages. A sample script
>>follows. Hard to tell without seeing your whole schema, but I see no
>>need for a relation_id primary key if you already have a unique
>>constraint on child_fk and parent_fk, so I made those into the
>>primary key for the relations table:
>
>
> Thank you for an excellent answer. I think I will have to study your
> code for a while. But is it such a bad idea to have a separate column
> for the primary key here? I see that there are two schools on this,
> with diametrically opposed views. For my own part, I feel that it at
> least doesn't hurt to have a surrogate key. Secondly, a single key
> value is easier to reference from another table than a composite key.

Both are true and as another responder has noted, there are times when
surrogate keys are appropriate.

Be aware, though, that the real danger is data integrity. Should the
alternate key on your composite key get "accidentally" dropped, invalid
data (logical duplicates) can now be inserted.

--
Daryl

"We want great men who, when fortune frowns, will not be discouraged."
-- Colonel Henry Knox, 1776