Geography Reference
In-Depth Information
Figure 3.19: Search Query Builder
To preview a sample of the data, use the Sample button. This will pull
a subset of the unique values for the field. Viewing a sample is a good
method to use when the attribute table is very large because it saves
time. If you want to see all possible values for a field, use the All button.
These two boxes also serve another purpose. You can add an item in
the box to the query by double-clicking it. To start building our sam-
ple query, we double-click the POPULATION field to add it to the SQL
where clause. To complete the population part of the query, we click the
“greater than” operator button and then type in “2000000.” This gives
us the following:
POPULATION > 2000000
When adding field names and operators, QGIS automatically inserts
the needed spaces. If we test the query now using the Test button, it
reports 110 matching features (cities). To complete the query, we click
the CAPITAL field and click the ALL button to get a list of all possible
values. It's easy to see there are two possibilities—Y and N. Since we are
using a value from the list, we don't have to type anything to complete
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