Indicative or subjunctive after "non sapere che"

Question

Must the verb in the dependent clause after the verb "sapere" in negative form be conjugated in the indicative mood or in the subjunctive mood? Different sources on Internet disagree on it. IMHO both would make sense; it depends if the focus is on the certainty of not knowing something or on the uncertainty about the subject of knowledge. Therefore, I think that the choice is idiomatic. Example for discussion:

• Non so che cosa vogliono. (present tense of indicative mood)
• Non so che cosa vogliano. (present tense of subjunctive mood)

Answer 1

This is another case of proposizione interrogativa indiretta, which I already talked about in this answer. Without repeating myself, both forms are acceptable although I personally prefer the second one.

The distinction is not as much about certainty/uncertainty but formal/informal language (the second one is ever so slightly more formal than the first one).

It is maybe worth mentioning that the usage of the subjunctive mood is preferred when the main clause is negative, so even people that tend to use the indicative might use the subjunctive in this context.