corrected SQL statement

- it's totally uneeded to join the FROM table once again
- I do not know about specifying multiple tables in one JOIN, it might be wrong, but at least it's (same as using CROSS JOIN syntax with multiple FROM tables) at least bad behaviour
- omitting the ON clause when using JOIN is afaik not optional in the SQL standard. MySQL allows it, but then uses CROSS JOIN. Here you either want NATURAL JOIN or JOIN with ON clause. I prefer JOIN with explicit ON clauses all the time (because I never call my columns Berater.berater_id, but Berater.id and in foreign key definitions then Foo.berater_id), but since in the lectures NATURAL JOIN was so often prefered, I thought about using that. In my own exam, I will always use ON clause, because I am more used to that and I will not run into troubles if there are two columns with the same name, which is exactly the problem here. There are two tables with column "name". The second NATURAL JOIN would fail, because it would try to not only JOIN Kunden2Berater.kunden_id with Kunden.kunden_id, but also Berater.name with Kunden.name.
Thus, I'd recommend to always use explicit ON statements.