Database Reference
In-Depth Information
The first step is to determine whether the INVENTORY table truly complies with the first
element of an ideal table (i.e., “It represents a single subject”). A table that contains a large
number of blank values in its fields usually—but not always—represents more than one
subject. Think about the two sets of fields in question for a moment, and you'll soon real-
ize that they represent characteristics of
two distinct aspects
of the table's subject. The first
setoffieldsdescribesequipmentinventory,andthesecondset of fields describes topics in-
ventory; furthermore, both types of inventory share common characteristics, such as I
TEM
N
AME
, I
TEM
D
ESCRIPTION
, and C
URRENT
V
ALUE
. In essence, “Equipment” and “Books” are
subjects that are dependent upon the INVENTORY table for their very existence; neither
describes a completely distinct object or event. As a result, they are
subordinate subjects,
and you'll create a
subset
table for each of them.
Justasadatatablerepresentsadistinctsubject,asubsettablerepresentsasubordinatesub-
ject of a particular data table. The subset table contains fields that are germane to the sub-
ordinate subject it represents, and it also includes a field (or fields) from the data table that
serves to relate the data table to the subset table. It's important to note that a subset table
does not
contain fields that represent characteristics common to both it and the data table;
these fields must remain in the data table.
Now that you've determined that the INVENTORY table describes three subjects (it
doesn'tmatterthattwoofthemaresubordinatesubjects),youmustbringitintocompliance
with the first element of an ideal table by removing the fields in question. You then use the
fields as the basis for two new subset tables, one for each subordinate subject. Here are the
steps you follow to accomplish these tasks.
1.
Use the M
ANUFACTURER
, M
ODEL
, and W
ARRANTY
E
XPIRATION
D
ATE
fields to create
a new subset table called EQUIPMENT.
2.
Use the P
UBLISHER
, A
UTHOR
, ISBN, and C
ATEGORY
fields to create a new subset
table called BOOKS.
3.
Add I
TEM
N
AME
to both tables; this field will relate each subset table to the data
table.
4.
Compose a suitable description for both subset tables and add them to the Final
Table List. Indicate each table's type as “Subset.”
Figure 7.29
shows the new subset table structures.
