Microsoft Office Tutorials and References
In Depth Information
You can see that, slicing for color on the rows, the first line contains all the products for which
there is no color defined. No color defined, in the technical world, means that the column color
does not contain any value. The lack of a value produces a report in which no label is attached
to the color, and this condition is not user-friendly.
The report would look much better if you were able to present data as you see it in Figure 4-17.
FIguRE 4-17 Using default values for empty ones, the report looks much more readable.
In this latter report, all rows have a label and, if no color has been supplied for specific products,
the report shows a value indicating <NO COLOR>. From the user’s perspective, this second
report is much more informative than the first one.
Technicians normally refer to empty values as NULL. NULL is a special value used to mean
missing . In other words, a column containing the special value NULL indicates a column for
which no value has been specified because that column was an optional one. In the technical
database, NULL values are useful because they do not waste space for missing values. In the
logical database, it would be much better to avoid NULL values. Whenever you have a value
which has not been specified, you want to show to the user a meaningful message, such as
NO COLOR, NO PRODUCT, or something similar that clearly helps the reader understand
the meaning of the message.
NULL values are very easy to handle, and for this reason, we prefer to show the technique
right here. If you look at the product table, as in Figure 4-18, you can note that the Color
column contains some color definition and some empty values. You now know that the
empty values correspond to the special value NULL.