Database Modeling and Design

6.3 Normalization of Candidate Tables Derived from ER Diagrams 119
Table 6.1
Primary FDs Derivable from ER Relationship Constructs
Degree Connectivity Primary FD
Binary or one-to-one 2 ways: key(one side) -> key(one side)
Binary one-to-many key(many side) -> key(one side)
Recursive many-to-many none (composite key from both sides)
Ternary one-to-one-to-one 3 ways: key(one), key(one) -> key(one)
one-to-one-to-many 2 ways: key(one), key(many) ->
key(one)
one-to-many-to-many 1 way: key(many), key(many) ->
key(one)
many-to-many-to-many
none (composite key from all 3 sides)
Generalization none none (secondary FD only)
not include nonkey attributes used in secondary FDs, the data requirements
specification or data dictionary must be consulted. Table 6.1 shows
the types of primary FDs derivable from each type of ER construct.
Each candidate table will typically have several primary and secondary
FDs uniquely associated with it that determine the current degree of
normalization of the table. Any of the well-known techniques for
increasing the degree of normalization can be applied to each table to
the desired degree stated in the requirements specification. Integrity is
maintained by requiring the normalized table schema to include all data
dependencies existing in the candidate table schema.
Any table B that is subsumed by another table A can potentially be
eliminated. Table B is subsumed by another table A when all the
attributes in B are also contained in A, and all data dependencies in B
also occur in A. As a trivial case, any table containing only a composite
key and no nonkey attributes is automatically subsumed by any other
table containing the same key attributes, because the composite key is
the weakest form of data dependency. If, however, tables A and B represent
the supertype and subtype cases, respectively, of entities defined by
the generalization abstraction, and A subsumes B because B has no
additional specific attributes, the designer must collect and analyze additional
information to decide whether or not to eliminate B.
A table can also be subsumed by the construction of a join of two
other tables (a “join” table). When this occurs, the elimination of a sub-