On 02/15/11 2:01 PM, Alpha Beta wrote:
> Hi,
>
> while reading about databases, I didn't understand what can be subset
> of attributes, see the following :
>
> Let Ri (Xi) be a relation sheme, where Xi is a set of attributes. and
> if t is a tuple for Ri (Xi) and Y is a subset of Xi, then t[Y] denote
> the subtuple of t corresponding to Y.
>
> does a subset of attributes just means the instances that exist in
> each tuple?
this question really has nothing to do with postgresql, except in the
most theoretical sense. perhaps you should find a forum or list that
deals with abstract database theory.