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.
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