Databases Reference
In-Depth Information
8.
From the
Expressions
dialog window, assign the
# of Carriers
label to
our expression.
9.
Click on
Finish
and we will be left with the following chart:
According to the table we see in the earlier screenshot, there are
41
carriers that fall
into the two-route category.
32
more serve a single interstate route,
20
other serve
4
interstate routes, and so on.
To design such a table, as you just saw, it's not necessary to create any pre-
aggregations in the source tables, nor is it required to have an "
Interstate Routes
"
field per se. The chart, along with its calculated dimension, is completely self-
contained and does not require other objects to function.
In an upcoming section, we'll extend on the nested aggregation
topic to group dimension values with the use of ranges.
A word on calculated dimensions
As useful as they are, calculated dimensions (such as the one we created earlier) are
not performance-friendly. Besides delaying calculation time, they can sometimes
prevent a chart's state from being cached to RAM, hence stopping QlikView's
caching algorithm from coming into play.
As calculated dimensions are sometimes necessary for advanced aggregations, it
is advisable to use this feature only when there is no other way of accomplishing
certain visualizations. Whenever a calculated dimension can be created as a new
field from the script, it is advisable to do so in order to use it in a more natural way
in chart objects.
