I have written a SQL query against the Stack Exchange Data Explorer that identifies answered questions with a positive score for which all the answers have a negative score (this was an answer to this Meta Stack Exchange post, though the code in that post is different from what I've posted below).
It's a fairly straightforward query; you can see it and run it on the Data Explorer (where you can also examine the schemata of the tables), and I have reproduced it below for your convenience.
SELECT q.Id AS [Post Link], -- this alias makes the Data Explorer do magic link rendering MAX(q.Score) AS [Score], COUNT(a.Id) AS [AnswerCount], AVG(CAST(a.Score AS float)) AS [AvgAnswerScore] FROM Posts AS q -- INNER JOIN => At least one answer INNER JOIN Posts AS a ON q.Id = a.ParentId WHERE -- Question has a positive score q.Score >= 1 GROUP BY q.Id HAVING -- Zero answers with a non-negative score SUM((CASE WHEN a.Score >= 0 THEN 1 ELSE 0 END)) = 0 ORDER BY [Score] DESC, [AnswerCount] DESC
While I of course would love to hear anything you have to say about this code, I am particularly interested in the following:
- Stylistic tips - coming from the perspective of a PEP8-adhering Python guy, the huge inhomogeneity in SQL coding styles makes me queasy.
MAX(q.Score)looks really stupid and is semantically wrong, since there's obviously only one
q.Idsince they come from the same table, and
q.Idis the primary key of that table. But putting
q.Scoreinto the GROUP BY also seems weird to me, since that's not semantically what I'm doing either. Is there a better way to approach this?
How do the performance characteristics of the HAVING clause here differ from instead using a WHERE clause of the form below?
q.Score >= 1 AND NOT EXISTS ( SELECT 1 FROM Posts AS a2 WHERE a2.ParentId = q.Id AND a2.Score >= 0 )
My sense is that
HAVING SUM(...) = 0is probably better, since, uh... there doesn't need to be a new subquery for each row of the outer query? But I don't know if that is true or even makes sense.