Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: paul c <toledobythesea_at_oohay.ac>
Date: Thu, 07 Jul 2005 21:06:13 GMT
Message-ID: <9Pgze.1894554$6l.1827902@pd7tw2no>

Jan Hidders wrote:
> 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

if the PK in R was 'entered wrong', wouldn't the rows in S be wrong anyway?

p Received on Thu Jul 07 2005 - 16:06:13 CDT