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I have one data set which contains all federal states of Germany. The other dataset is about the population density in every km² in the EU.

My goal is now to join these two Datasets so I can answer different questions like 'how many km² with more than 5000 people are in federal state x?'

The first data set looks like this:

The first data set looks like this

and the second like this:

like this.

I am very new to GIS programs, but from my intuition it sounds like a very basic need to combine different datasets in order to answer a question.

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    Welcome to gis.stackexchange! Please note that a good question on this site is expected to show some degree of research on your part, i.e. what you have tried so far. For more info, you can check our faq. – underdark May 31 '15 at 17:00
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    Of course I googled this question before and I could find out that two datasets can only be joined if they have a common key. This is not the case in this example unfortunately. But I could not find a possibility to answer the question. I just need a little keyword so that I can go on. – OST_EE May 31 '15 at 17:03
  • Are the datasets you're working with spatial (i.e. with geometries)? If yes, which format? – underdark May 31 '15 at 17:30
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    If you have two Shapefiles, you can compute the intersection gis.stackexchange.com/questions/tagged/qgis+intersection but make sure that both datasets are in the same projection before you perform the intersect. – underdark May 31 '15 at 18:30
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    Intersection is a spatial property, not an attribute property. You cannot join tables without a common key, but you can join or perform spatial operations on them if they have their own geometry. Based on your description of the data, there should be intersection (overlap) spatially between the two files. Shapefiles are actually composed of several files. The .shp is the one you want to add to QGIS/work with. This blog post describes some basic overlay operations. – Chris W May 31 '15 at 20:11
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I guess your dataset about population density is the GEOSTAT grid dataset from Eurostat. I have already worked with it so I can give you the main steps.

First, your GEOSTAT dataset should come with a shapefile (for me, it's called Grid_ETRS89_...). You have to import in QGIS both this shapefile and the .csv file that contains the population data.

Then, you have to perform a join between these two files to link the population information to the spatial information. The join will be done on the field "CODE" of your .csv.

After that (assuming your Länder dataset has the same datum than the Eurostat dataset), you'll just have to use a tool like Intersect (in Vector > Geoprocessing) to select all 1 km² squares inside a specific Land.

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