Database Reference
In-Depth Information
3.
This structure is inherently volatile.
In its current state, the table unnecessarily re-
stricts the number of categories an instructor can teach; you must create additional
category fields when you have instructors who teach more than three categories.
Adding more category fields just compounds the first two problems.
RealizingthatflatteningtheC
ATEGORIES
T
AUGHT
fieldwon'tsolveyourproblem,yournext
thought is to bring the field into compliance with the second element of an ideal field and
declare that it will contain only a single value. Although this is a good impulse and a step
in the right direction, it will not resolve the matter completely because it will introduce yet
another problem: data redundancy.
Figure 7.19
illustrates what happens when you follow
through with this particular idea. Note that there is now a single value in the C
ATEGORIES
T
AUGHT
field for each record in the table.
Figure 7.19. The result of bringing C
ATEGORIES
T
AUGHT
into compliance with the
second element of an ideal field
The values in C
ATEGORIES
T
AUGHT
cause redundant data because you must duplicate a giv-
en instructor record
for each category
that the instructor teaches. This redundancy is obvi-
ously unacceptable, so you'll have to resolve this problem in some other manner.
You can avoid this situation entirely by using these steps to resolve a multivalued field.
1.
Remove the field from the table and use it as the basis for a new table. If neces-
sary, rename the field in accordance with the field name guidelines that you
learned earlier in this chapter.
