Free and Open Source GIS Ramblings

Aggregate all the things! (QGIS expression edition)

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In the past, aggregating field values was reserved to databases, virtual layers, or dedicated plugins, but since QGIS 2.16, there is a way to compute aggregates directly in QGIS expressions. This means that we can compute sums, means, counts, minimum and maximum values and more!

Here’s a quick tutorial to get you started:

Load the airports from the QGIS sample dataset. We’ll use the elevation values in the ELEV field for the following examples:

QGIS sample airport dataset – categorized by USE attribute

The most straightforward expressions are those that only have one parameter: the name of the field that should be aggregated, for example:

mean(ELEV)

We can also add a second parameter: a group-by field, for example, to group by the airport usage type, we use:

mean(ELEV,USE)

To top it all off, we can add a third parameter: a filter expression, for example, to show only military airports, we use:

mean(ELEV,USE,USE='Military')

Last but not least, all this aggregating goodness also works across layers! For example, here is the Alaska layer labeled with the airport layer feature count:

aggregate('airports','count',"ID")

If you are using relations, you can even go one step further and calculate aggregates on feature relations.

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