The data query is looking for question marked as unanswered, with no upvoted answer, under a specific tag, for e.g. pde.
While I was doing this earlier this evening, I found that actually some of the questions obtained through this query actually had an upvoted answer. For example:
Then I was going to edit the query, and found there is nothing wrong with the predicate:
p.AcceptedAnswerID IS NULL AND
p.ClosedDate IS NULL AND
p.Score >= 0 AND
a.Score = 0 AND
a.Score >= ALL
Is this a bug of the SE data explorer?