T-SQL Multi-layered CTE query with Aggregates - sql-server

Question

I have a long Common Table Expression (CTE) query which is trying to calculate percent difference between each users' average score and group average score.
I would like for my multi-layered CTE query to filter and reduce bulk of records down to the following table:
UserID Tag UserAvg GroupAvg PercentDifference
1 Cat 72.50 73 -0.68
2 Cat 75.50 73 3.36
3 Cat 75 73 2.70
4 Cat 73.25 73 0.34
5 Cat 52.3333 73 -32.97
6 Cat 86.25 73 16.64
My problem is getting GroupAvg column so that I can perform % Difference calculation.
To illustrate the current approach I am using; here is the summary of my CTE query:
WITH
-- select 1st 3 columns
UserScores AS (select UserID, Tag, Score FROM {multiple-table} WHERE Tag = 'Cat'),
-- add UserAvg column by grouping records
ScoreAverages AS (select UserID, Tag, AVG(Score) AS UserAvg GROUP BY UserID, Tag FROM UserScores),
-- calculate GroupAvg
GroupAverage AS (select AVG(UserAvg) AS GroupAvg FROM ScoreAverages),
-- calculate % difference
PercentDiff AS (select UserID, Tag, UserAvg, 73 AS GroupAvg, (((UserAvg-73)/((UserAvg+73)/2))*100) AS PercentDifference FROM ScoreAverages )
-- do something with results
select * from PercentDiff
Simple enough; right?
Notice that I have hard coded 73 as my GroupAvg value. I am unsure how to construct required sql query that would allow me to go from ScoreAverages to PercentDiff table.
Is it possible to perform SELECT within a SELECT statement? And I am not looking for something of the following:
select * from X where Id in (select Id from Y where Name like '%abc%')
Or I am simply trying to do too much in one go?

Answer 1

Yes, it's called a sub-select:
SELECT Column1, Column2, (SELECT QUERY THAT GETS GROUP AVERAGE) AS GroupAverage, Column3
FROM ...
To use the result of the sub-select in another column's calculation, you can either repeat the sub-select:
SELECT Column1, Column2, (SELECT QUERY THAT GETS GROUP AVERAGE) AS GroupAverage, (Column3 - (SELECT QUERY THAT GETS GROUP AVERAGE)) AS Column4
FROM ...
Or you can reference it the same as you would any other column in the outer query or a subsequent CTE:
WITH CTE1 AS (SELECT Column1, Column2, (SELECT QUERY THAT GETS GROUP AVERAGE) AS GroupAverage
FROM ...)
, CTE2 AS (SELECT *, Column3-GroupAverage) AS Column4
FROM CTE1
JOIN ...

Answer 2

It is possible, as shown in Tab Alleman's answer, but in your case it's not necessary. Since you already calculate the GroupAvg in the cte chain, you can use it in the final query. and since the GroupAverage only contains one row, you can simply add a CROSS JOIN to it:
;WITH
-- select 1st 3 columns
UserScores AS (
select UserID, Tag, Score
FROM {multiple-table}
WHERE Tag = 'Cat'),
-- add UserAvg column by grouping records
ScoreAverages AS (
select UserID, Tag, AVG(Score) AS UserAvg
FROM UserScores
GROUP BY UserID, Tag),
-- calculate GroupAvg
GroupAverage AS (
select AVG(UserAvg) AS GroupAvg
FROM ScoreAverages),
-- calculate % difference
PercentDiff AS (
select UserID, Tag, UserAvg, GroupAvg,
(((UserAvg-GroupAvg)/((UserAvg+GroupAvg)/2))*100) AS PercentDifference
FROM ScoreAverages
CROSS JOIN GroupAverage)
-- do something with results
select * from PercentDiff

Answer 3

I just thought you could do this with a single cte like so.
;WITH UserAverages AS
(
SELECT UserID,
Tag,
AVG(Score) AS UserAvg,
AVG(AVG(Score)) OVER () AS GroupAvg
FROM {multiple-table}
WHERE Tag = 'Cat'
GROUP BY UserID, Tag
)
SELECT UserID,
Tag,
UserAvg,
GroupAvg,
(((UserAvg-GroupAvg)/((UserAvg+GroupAvg)/2))*100) AS PercentDifference
FROM UserAverages