Tom Kelliher, CS 318
Apr. 3, 2002
Read 9.1--3.
Normal forms; properties of decompositions.
Triggers.
Minimal covers come close.
Note: Just apply FD decomposition.
Note on computation: To check to see if in
of
is redundant, check
whether or not
is entailed by
.
Note on computation: To check to see if an FD of is redundant,
create
which is just
without the candidate
redundant FD. Check whether or not the candidate FD is entailed by
.
Otherwise, let be a key of
. Add
to the schema set.
Why is the resulting schema
Consider the following schema :
and
contains
This will synthesize into two relations. The third FD actually will apply to both relations.
Recall, the 3NF will be dependency preserving. A dependency preserving BCNF schema is our first choice.
Decomposition :=
while there is a schema in
Decomposition that is not in BCNF do
/* Assume in
violates BCNF. */
Replace in Decomposition with schemas
and
where
and
are all the FDs of
that involve only
attributes from their respective schemas
end
Questions: