June 2017
Beginner
428 pages
10h 2m
English
Content preview from Practical GIS
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Writing advanced queries
We discussed earlier that basic SQL expressions in GIS allow us to select features in only one layer. However, QGIS offers us numerous advanced options for querying even between layers. Unlike filtering, the query dialog allows us to use this extended functionality. These operations are functions which require some arguments as input and return values as output. As we can deal with many kinds of return values, let's discuss how queries work in QGIS.
First, we build an expression. QGIS runs the expression on every feature in the queried layer. If the expression returns true for the given feature, it becomes selected or processed. We can test this behavior by opening the query dialog and simply typing TRUE. Every feature ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.