qgisqgis-2.18symbology
I want to combine categorized symbol data for the sake of symbology. For example: Let's say I classified a data column of a shapefile and it gave me 20 different values. Ten commercial building types and ten residential building types. In Arcgis I could very easily combine that to two symbols, one for residential and one for commercial.
Running QGIS 2.18, MacOS Sierra 10.12.2
In the new symbology choose Graduated then define a new color ramp, by choosing Color ramp option and going down on the menu. When you define a new color ramp you have the option Random. Then you can make some settings on the number of classes and the colors and then save the color ramp. After you created the random color ramp you can choose the number of classes equal with the number of colors defined on the ramp and click on classify. You can repeat the workflow until you have the desired colors.
As @MichaelMiles-Stimson already mentioned, there doesn't seem to be a way to symbolise NULL features. However, there is an alternative whereby you create a filter to force QGIS to treat NULL values as an integer such as 0. I've included an example where I created 3 simple polygons each with a certain value:
Here is the Graduated Symbology I used with the following command:
case when "Some_Value" IS NULL then 0 else "Some_Value" end
Hope this helps!
Best Answer
The way to go is to use Rule-based symbology (as stated at the end of previous answer)
You may have to adjust the setting to fit your needs, the expression in the picture assume that you use numerical value for classification and you will need to create another rule for residential building.