Database Reference
In-Depth Information
Figure 7.21
shows a version of the INSTRUCTORS table that contains
three
multivalued
fields.
C
ATEGORIES
T
AUGHT
—This indicates the categories of classes that an instructor can
teach.
M
AXIMUM
L
EVEL
T
AUGHT
—Thisindicatesthemaximumskilllevelthattheinstruct-
or can teach for a given category.
L
ANGUAGES
S
POKEN
—This indicates the foreign languages that an instructor can
speak.
Figure 7.21. A version of the INSTRUCTORS table containing three multivalued
fields
Your task here seems relatively clear—you're going to use the procedure you've just
learned to resolve these multivalued fields. You then notice one small, relatively obscure
problem: There is a distinct one-to-one association between values in C
ATEGORIES
T
AUGHT
and the values in M
AXIMUM
L
EVEL
T
AUGHT
for any given record. You probably wouldn't
have noticed this anomaly had you not carefully examined the sample data within these
fields. Don't worry; you'll still use the same procedure, but with one minor modification.
You'll occasionally encounter a situation such as this, where some given field (whether
single- or multivalued) depends on a particular multivalued field. You can easily fix this
problem by including the dependent field in the structure of the new table you build to
resolve the multivalued field.
Figure 7.22
shows the results of consolidating this tech-
nique with the previous one to resolve C
ATEGORIES
T
AUGHT
. (It shows the resolution of
L
ANGUAGES
S
POKEN
as well.)
