问如何从Firebase中的多个节点检索数据？安卓

EN

DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("users");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for (DataSnapshot userSnapshot: dataSnapshot.getChildren()) {
            DataSnapshot questionsSnapshot = userSnapshot.child("questions");
            Log.i(TAG, "User "+userSnapshot.getKey()+" has "+questionsSnapshot.numChildren()+" questions");
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {
        throw databaseError.toException();
    }
}