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

paul c schrieb:
> 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.
>>

Update and delete-insert should be fundamentally different operations for the following reason. Update deals with object semantics, i.e., how they are characterized by other *existing* objects. In particular, update does not change the number of objects and their references. Delete-insert is a life-cycle management operation that has nothing to do what objects mean. Instead, it creates or deletes references, which reflect object existence. As I already mentioned somewhere in this forum we need to distinguish two layers:
- object semantics (field values), and
- object representation (references).
If they are not separated like in the RM then there is no need to separate the operations of update and delete-insert (at fundamental level). Thus in the RM, I agree, these two operations are quite comparable. But in this case we have numerous problems. One of them is updating primary key (which has to be disabled in a good model). However, if we want to separate these two layers then these operations are fundamentally different.

-- 
http://conceptoriented.com