如何创建对arango db NOT NULL的正确请求？

Question

我有“ sub”（订阅）。它具有属性'removed'-当它为null时，sub是活动的，而当它不为null时-它已去除。以下查询正确吗？

FOR sub IN Subscription
COLLECT state = CANCELED,
        removed != null,
        WITH COUNT INTO cnt
SORT cnt DESC
RETURN {"Count of removed subscriptions":cnt}