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Marshall wrote:
> ...
Just curious, regarding different constraints than keys,
R(a, b)
{a} -> {b} =def=
forall R(a, b): forall R(a', b'): a = a' AND b = b'
constrain R to a value that has at most one row?
2) Does
R(a, b)
{a} -> {b} =def=
forall R(a, b): forall R(a', b'): a = a' OR b = b'
constrain R to a value that has either zero rows or some number of rows that have the same value for one of a or b?
(If so, I suppose a and b could be seen as "alternate" keys, which one becomes key depends on the first two "insertions". Have I got that wrong?)
3) Does
R(a, b)
{a} -> {b} =def=
forall R(a, b): forall R(a', b'): NOT a = a' AND NOT b = b'
constrain R to a value has zero rows?
p Received on Sat Feb 24 2007 - 09:58:19 CST