Re: subset of attributes

Some context would help but some (possibly relevant) possibilities:

1: If you have relation with 5 attributes you can generate tuples with all 5
attributes OR you can generate tuples with a subset of those attributes.
Seems obvious but often these kinds of definitions are.

2: When looking at a relation you want to consider whether any subset of the
attributes are dependent upon another attribute that is not a key for the
relation. If so then that subset can likely be broken out into its own
relation with the attribute that the subset relates to acting as a foreign
key on the original relation.

Without further context as to where and why the definition is being given
further conclusions are tough to draw. My first point is simply an obvious
property of how relations and attributes and the second relates to
normalization of a relation schema.

My $0.03 (inflation ya know)

David J

From: pgsql-general-owner(at)postgresql(dot)org
[mailto:pgsql-general-owner(at)postgresql(dot)org] On Behalf Of Alpha Beta
Sent: Tuesday, February 15, 2011 5:01 PM
To: pgsql-general(at)postgresql(dot)org
Subject: [GENERAL] subset of attributes

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?

Best,