Databases Reference
In-Depth Information
Dealing with dimensions without facts
Sometimes, a dimension table can contain values that do not have any associated
facts. To demonstrate this, let's take a second look at the data model we built in
Chapter 3, Data Sources,
and have been using ever since:
1.
Open the
Airline Operations.qvw
ile.
2.
Launch the Table Viewer by selecting
File | Table Viewer
or by pressing
Ctrl
+
T
.
3.
Hover the mouse over the
%Aircraft Type ID
field in the
Aircraft Types
table, pay special attention to the
Subset ratio
value.
4.
Next, hover the mouse over the
%Aircraft Type ID
field in the
Main
Data
table, again paying special attention to the
Subset ratio
. What you
will notice is that the
Aircraft Types
dimension table has a subset ratio
of
100%
for the field
%Aircraft Type ID
, while the
Main Data
table
only has a
48%
subset ratio, seen here:
We learned earlier what this means: of all the distinct possible values for
%Aircraft
Type ID
,
100%
of those values appear in the
Aircraft Types
dimension table, while
only
48%
of the values appear in the
Main Data
table. In other words, only
48%
of
aircrafts have actually made any flights.
Before we look at how to remove these aircraft types from the model, let's first
quickly investigate which aircraft types have not logged any flights:
1.
Close the
Table Viewer
window by clicking on
OK
.
2.
Add a new sheet to the document by clicking on the
Add Sheet
button from
the
Design
toolbar.
3.
Once the new sheet is created, right-click on the workspace area and select
Properties…
. Then, from the
Sheet Properties
window, activate the
General
tab and change the
Title
field to
Data Consistency
.
