Re: deductive databases

From: alex goldman <hello_at_spamm.er>
Date: Thu, 12 May 2005 15:55:37 -0700
Message-Id: <3907907.uYUagk22Ws@yahoo.com>

Jan Hidders wrote:

> alex goldman wrote:

>> While people who responded seem to disagree on whether SQL has recursion,

>
> ?? I didn't see any disagreement. What the different answers told you
> was that this differs per SQL standard and per implementation.
>

>> what about functors?
>> 
>> For example, can you express something like this with SQL?
>> 
>> for_any X Y : car(cons(X, Y), X)

>
> That depends on what you mean by "can express". Since SQL is a query
> language in which you formulate queries over tables it can only
> formulate queries over tables and not over functors. So in that sense
> the answer is "no" but that observation is about as interesting as the
> fact that SQL also cannot make coffee. If you reformulate it as a
> statement about tables by, for example, modeling car as a binary table
> and cons as a ternary table then you can express this and for that you
> don't even need recursion.

The difference between having functors and not having them is fundamental. In one case inference is decidable, and in another it isn't (If you think this is uninteresting, you are probably in the wrong business) The expressiveness varies accordingly. Received on Thu May 12 2005 - 17:55:37 CDT