I have started with knime and I do the free certificate course.
There in the solution is a “DB Selector Node” used but I can not find this one. Does anyone know where it is? It must be an extension I think… . 
@michael390 welcome to the KNIME Forum. The selector is here:
You can also search for it in the Nodes section:
You might have to activate “All nodes” in the Filter configuration.
thank you very much, this “All nodes” was the solution 