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Ordering of tuples in a relation r(R): The tuples
are not considered to be ordered, even though they appear to be in the tabular
form.
Ordering of attributes in a relation schema R (and
of values within each tuple): We will consider the attributes in R(A1, A2, ...,
An) and the values in t=<v1 , v2, ..., vn> to be ordered .
(However, a more general alternative definition of
relation does not require this ordering).
Values in a tuple: All values are considered atomic
(indivisible). A special null value is used to represent values that are
unknown or inapplicable to certain tuples.
Notation:
We refer to component values of a tuple t by t[Ai] =
vi (the value of attribute Ai for tuple t). Similarly, t[Au, Av, ...,Aw] refers
to the subtuple of t containing the values of attributes Au, Av, ..., Aw, respectively.
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