Re: foundations of relational theory?

Jonathan Leffler wrote:

> cmurthi wrote:
>
>> Marshall Spight wrote:
>>
>>> I'd still argue that a declarative integrity enforcement
>>> system is better than a procedural one.
>>
>>
>> Can a declaration enforce a really complex oonstraint? And if so,
>> does the declaration begin to look like a procedural statement? For
>> example, even on a simple level, how would you declare that Field1
>> has to be conditionally based on Field2 and Field3, eg, procedurally,
>> if(field3=='') field1=field2 else field1=field3.
>
>
> Using the Informix dialect of SQL, and ignoring SQL NULL values (since
> they make a mess of everything), you could do:
>
> ALTER TABLE WhatNot ADD CONSTRAINT ((field3 = '' AND field1 = field2)
> OR (field3 != '' AND field1 = field3));
>
> You could give the constraint a name - though ISO SQL and Informx
> manage to do that differently. You could also include that
> declarative constraint in the CREATE TABLE statement.
>
> I don't regard that as a particularly complex constraint. I'm not
> sure that it is part of a good database design, but that is probably
> simply because it is illustrative rather than anything else.
>
> The big advantage of the declarative constraint is that the DBMS
> enforces it. No application can violate it, whether accidentally or
> on purpose. If the constraint becomes obsolete, then you drop it.
> Altering it is a two-stage drop and add operation (which Informix
> permits you to do in a single ALTER TABLE statement, so there is no
> window of vulnerability while you are changing the constraint).

The problem with constraints like these is that, yes, you safeguard your db from corrupt data, but getting the information back into the application is not that easy: you have to parse the error message, get the name of the constraint and translate it into a meaningfull error message for the user. I don't know if anybody does this, so, in effect you end up coding the same checks twice: in the app and in the db.

This is where we run into the frustrating "mismatch" between app language and db environment.

regards,
Lauri

>
>
Received on Tue Oct 28 2003 - 21:41:53 CET