SE Data Explorer Query: Average score for questions and answers, by tag Being curious to see which tags on this site get the most attention, I developed a Stack Exchange Data Explorer query:

WITH CanonicalTags AS (
    SELECT Id, TagName, Id AS AliasId, TagName AS Alias
        FROM Tags
        WHERE NOT EXISTS (
            SELECT Id
                FROM TagSynonyms
                WHERE SourceTagName=TagName AND ApprovedByUserId IS NOT NULL
        )
        UNION
    SELECT Tags.Id, Tags.TagName, Alias.Id AS AliasId, Alias.TagName
        FROM Tags
            INNER JOIN TagSynonyms
                ON TargetTagName=TagName
            INNER JOIN Tags AS Alias
                ON Alias.TagName=SourceTagName
        WHERE ApprovedByUserId IS NOT NULL
), TagQuestionStats AS (
    SELECT Tags.Id AS Id
         , Tags.TagName AS TagName
         , AVG(CAST(Questions.Score AS DECIMAL)) AS AvgScore
         , STDEV(Questions.Score) AS StdDevScore
         , COUNT(Questions.Id) AS N
        FROM CanonicalTags AS Tags
            INNER JOIN PostTags
                ON PostTags.TagId=Tags.AliasId
            INNER JOIN Posts AS Questions
                ON Questions.Id=PostTags.PostId
        GROUP BY Tags.Id, Tags.TagName
), TagAnswerStats AS (
    SELECT Tags.Id AS Id
         , Tags.TagName AS TagName
         , AVG(CAST(Answers.Score AS DECIMAL)) AS AvgScore
         , STDEV(Answers.Score) AS StdDevScore
         , COUNT(Answers.Id) AS N
        FROM CanonicalTags AS Tags
            INNER JOIN PostTags
                ON PostTags.TagId=Tags.AliasId
            INNER JOIN Posts AS Questions
                ON Questions.Id=PostTags.PostId
        INNER JOIN Posts AS Answers
            ON Answers.ParentId=Questions.Id
        WHERE
            Answers.PostTypeId=2
        GROUP BY Tags.Id, Tags.TagName
)
SELECT '&lt;' + QStats.TagName + '&gt;' AS [TagName]
     , ROUND(QStats.AvgScore, 2) AS [Avg Qst Score]
     , ROUND(QStats.StdDevScore, 2) AS [Qst Score Std Dev]
     , QStats.N AS [# Qst]
     , ROUND(AStats.AvgScore, 2) AS [Avg Ans Score]
     , ROUND(AStats.StdDevScore, 2) AS [Ans Score Std Dev]
     , AStats.N AS [# Ans]
     , ROUND(CAST(AStats.N AS DECIMAL) / CAST(QStats.N AS DECIMAL), 2) AS [Avg # Ans/Qst]
    FROM
        TagQuestionStats AS QStats
            LEFT OUTER JOIN TagAnswerStats AS AStats
                ON AStats.Id=QStats.Id
    ORDER BY 5 DESC, 2 DESC, 7, 4, 1;


I'd like to ask:


Some of the numbers don't round to two decimal places very well.  Is that due to a bug in SQL Server, or am I using inappropriate rounding techniques?  (The exact numbers don't matter; I'd much rather have a compact presentation.)
Is there an easier way to canonicalize the tags?
Any comments about my SQL formulation in general?
Any suggestions for making the query itself more meaningful?
Your code is, for the most part, consistently styled, and predictably structured, which is nice. I am not a fan of the comma-on-the-new-line style of continuation. I prefer the comma at the end. But, at least you are consistent with this.

One inconsistency which I see is that for smaller selects you have multiple selected columns on a single line.

SELECT Tags.Id, Tags.TagName, Alias.Id AS AliasId, Alias.TagName
    ....


To be consistent with the other selects, as much as these are short statements, you should maintain consistency:

SELECT Tags.Id
     , Tags.TagName
     , Alias.Id AS AliasId
     , Alias.TagName
   ....


There is only one example of it in your code, but I really dislike multiple conditions on a single line:

WHERE SourceTagName=TagName AND ApprovedByUserId IS NOT NULL


This really should be:

WHERE SourceTagName=TagName
  AND ApprovedByUserId IS NOT NULL


The UNION condition is also very buried in there, and it has too much indentation ;-)

Finally, white-space is cheap in SQL, I really don't like the space-less arg=val expressions, spacing them is easy arg = val.

Right, that's the nit-picky stuff done.

Talking about the UNION, the statement could be rewritten as:

SELECT IsNull(Main.TagName, Alias.TagName) AS TagName
     , IsNull(Main.Id,      Alias.Id     ) AS Id
     , Alias.Id                            AS AliasId
    FROM Tags AS Alias
    LEFT JOIN TagSynonyms 
            ON SourceTagName = Alias.TagName
    LEFT JOIN Tags AS Main
            ON TargetTagName = Main.TagName


I prefer the more compact result, although it is debatable as to whether the "Outer" Join is a better solution than the UNION. Outer joins can lead to confusion. So can UNIONS and I prefer the outer join.

In the above query I have removed the Alias TagName since it is not used outside the sub-select.

I also found the final 'assimilation' of all the data in the final select to have too many casts and data conversions. The actual logic was lost in the detail. I moved the complicating CASTS to the previous with clauses.

Finally, the indexed ORDER BY has caught a number of people out in the past, as it is too easy to change columns and suddenly the data ordering is off, even though the query still runs. I always recommend named-column ordering.

Putting this all together, I have the SQL:

WITH CanonicalTags AS (
    SELECT IsNull(Main.TagName, Alias.TagName) AS TagName
         , IsNull(Main.Id,      Alias.Id     ) AS Id
--         , Alias.TagName                       AS AliasName
         , Alias.Id                            AS AliasId
        FROM Tags AS Alias
            LEFT JOIN TagSynonyms 
                ON SourceTagName = Alias.TagName
            LEFT JOIN Tags AS Main
                ON TargetTagName = Main.TagName
), TagQuestionStats AS (
    SELECT Tags.Id AS Id
         , Tags.TagName AS TagName
         , CAST(AVG(CAST(Questions.Score AS DECIMAL)) AS DECIMAL(8,2)) AS AvgScore
         , CAST(STDEV(Questions.Score) AS DECIMAL(8,2))  AS StdDevScore
         , CAST(COUNT(Questions.Id) AS DECIMAL(8,2))  AS N
        FROM CanonicalTags AS Tags
            INNER JOIN PostTags
                ON PostTags.TagId=Tags.AliasId
            INNER JOIN Posts AS Questions
                ON Questions.Id=PostTags.PostId
        GROUP BY Tags.Id, Tags.TagName
), TagAnswerStats AS (
    SELECT Tags.Id AS Id
         , Tags.TagName AS TagName
         , CAST(AVG(CAST(Answers.Score AS DECIMAL)) AS DECIMAL(8,2)) AS AvgScore
         , CAST(STDEV(Answers.Score) AS DECIMAL(8,2)) AS StdDevScore
         , CAST(COUNT(Answers.Id) AS DECIMAL(8,2)) AS N
        FROM CanonicalTags AS Tags
            INNER JOIN PostTags
                ON PostTags.TagId=Tags.AliasId
            INNER JOIN Posts AS Questions
                ON Questions.Id=PostTags.PostId
        INNER JOIN Posts AS Answers
            ON Answers.ParentId=Questions.Id
        WHERE
            Answers.PostTypeId=2
        GROUP BY Tags.Id, Tags.TagName
)
SELECT '&lt;' + QStats.TagName + '&gt;' AS [TagName]
     , QStats.AvgScore AS [Avg Qst Score]
     , QStats.StdDevScore AS [Qst Score Std Dev]
     , QStats.N AS [# Qst]
     , AStats.AvgScore AS [Avg Ans Score]
     , AStats.StdDevScore AS [Ans Score Std Dev]
     , AStats.N AS [# Ans]
     , ROUND(AStats.N / QStats.N, 2) AS [Avg # Ans/Qst]
    FROM
        TagQuestionStats AS QStats
            LEFT OUTER JOIN TagAnswerStats AS AStats
                ON AStats.Id=QStats.Id
    ORDER BY [Avg Ans Score] DESC
           , [Avg Qst Score] DESC
           , [# Ans]
           , [# Qst]
           , [TagName];


This query supports this fork of your original question