title: The difference between countDistinct and distinct.count
body: <p>Why do I get different outputs for <code>..agg(countDistinct("member_id") as "count")</code> and <code>..distinct.count</code>?
  Is the difference the same as between <code>select count(distinct member_id)</code> and <code>select distinct count(member_id)</code>?</p>