SQL exercises don't recognize wrong results
The comparison of the learner result with the sample result is always marked as correct.
To reproduce, check for example exercise 6 ("Bestimmen Sie den Titel des Buches mit der ISBN '978-3551354051' (ohne Anführungszeichen).").
Giving this solution, which is missing quotation marks around the ISBN, causes an empty result, but is marked correct:
SELECT title FROM books WHERE isbn = 978-3551354051
As the condition only contains numbers and a minus, it is interpreted as arithmetic expression and therefore valid SQL. The comparison of conditions also does not mark the solution without question marks as wrong.
In case of exercise 6, the instruction "(ohne Anführungszeichen)" caused confusion for students, as leaving them out of the actual query caused the answer to be accepted. I suggest to remove this part and the quotes in the instruction, as soon as result checking is fixed.