Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Thu, 07 Jul 2005 20:36:05 GMT
Message-ID: <Vmgze.139648$4j4.7330905_at_phobos.telenet-ops.be>
>
> like a few other people (i suspect we are a minority), i think of
> 'update' as a sugaring or shortcut. ignoring transaction or concurrency
> issues, is there any logical difference between 'update' and the
> combination of 'delete-insert'?
Date: Thu, 07 Jul 2005 20:36:05 GMT
Message-ID: <Vmgze.139648$4j4.7330905_at_phobos.telenet-ops.be>
paul c wrote:
> Jan Hidders wrote:
>> >> Another small thing is updating primary keys. If a primary key has >> accidentally been entered wrong and you want to fix that with an >> update then it is usually not possible to simply update it, and the >> problem gets even worse if it is also refered to by foreign keys. In >> an ER model this is a non-problem.
>
> like a few other people (i suspect we are a minority), i think of
> 'update' as a sugaring or shortcut. ignoring transaction or concurrency
> issues, is there any logical difference between 'update' and the
> combination of 'delete-insert'?
Splitting the update in a delete and an insert makes the situation even more complex. Assume a relation S that has a foreign key to the primary key of R. For an update to the PK of a tuple in R it is easy to see what it means to cascade that update to S. If you split the update in a delete and an insert this is not so simple because on the delete you would have to either (1) delete the corresponding tuples in S or (2) nullify their foreign keys, so if you then follow with only the insert you will have lost information. So, yes, there is a logical difference.
- Jan Hidders