On May 19, 2:20 pm, Vadim Tropashko <vadimtro_inva..._at_yahoo.com>
wrote:
> Grouping by the empty set of columns. The input
>
> A
> y
> -
> a
> b
>
> [set equality] joined with
>
> P
> y set
> - -----
> a {a,b}
> b {a,b}
> a {a}
> b {b}
>
> produces
>
> set
> -----
> {a,b}
>
> The empty relation A would naturally produce the output
>
> set
> ---
> {}
>
> Hence, my suggestion to define group by empty set to always produce
> one row.
Oh, I see the problem! There is no empty set listed in the powerset
relation. Therefore, indeed set equality join would produce the empty
relation...
Received on Sat May 19 2007 - 16:38:18 CDT
