Re: [ISSUE-41 and ISSUE-42] What do people think of NULLs


I very much appreciate your input. Now, I think I start to understand  
what we shouldn't do, but I don't understand what we should do, in  
your opinion. Can you please come up with a concrete proposal how to  
handle this?

Dr. Michael Hausenblas, Research Fellow
LiDRC - Linked Data Research Centre
DERI - Digital Enterprise Research Institute
NUIG - National University of Ireland, Galway
Ireland, Europe
Tel. +353 91 495730

On 18 May 2011, at 08:45, Enrico Franconi wrote:

> On 18 May 2011, at 06:52, Juan Sequeda wrote:
>> Seems like an unofficial consensus is to completely ignore NULL  
>> values.
> This is also not information preserving wrt the normative behaviour  
> in SQL.
> This shows up when there is an update: you can update a null value,  
> since it is compatible with the schema; on the other hand, you can  
> not update a missing value if there is no attribute in the schema.  
> The NULL values also affect aggregated queries, so they can not be  
> ignored.
> The SQL way is to introduce a special NULL constant *and* requiring  
> that the queries introduce an inequality test for all join variables  
> (which should never be equal to NULL). This introduces no additional  
> cost in terms of query answering complexity (indeed SQL NULL values  
> are efficient); it is rather expensive in terms of reasoning over  
> queries, e.g., query containment (see [1]), since it introduces  
> inequality in the queries.
> cheers
> --e.
> [1] Containment of Conjunctive Queries over Databases with Null  
> Values, by C. Farré and W. Nutt and E. Teniente and T. Urpí; < 
> >.

Received on Wednesday, 18 May 2011 08:04:05 UTC