Here's another way:
;WITH Preaggregate AS (
SELECT Grp, [Result] = SUM(numLow) + SUM(numMedium) + SUM(numHigh) + SUM(numZeroScore)
FROM historyTable h
CROSS APPLY (
SELECT [Grp] = CASE
WHEN h.asAtDate > DATEADD(DAY,-7,CAST(GETDATE() AS DATE)) THEN 'thisWeek'
WHEN h.asAtDate > DATEADD(DAY,-14,CAST(GETDATE() AS DATE)) THEN 'lastweek'
--WHEN h.asAtDate > DATEADD(DAY,-21,CAST(GETDATE() AS DATE)) THEN 'weekbeforelast'
ELSE NULL END
) x
--WHERE h.asAtDate > DATEADD(DAY,-21,CAST(GETDATE() AS DATE))
WHERE h.asAtDate > DATEADD(DAY,-14,CAST(GETDATE() AS DATE))
GROUP BY Grp
)
SELECT
[thisWeek] = CASE WHEN Grp = 'thisWeek' THEN Result ELSE 0 END,
[lastweek] = CASE WHEN Grp = 'lastweek' THEN Result ELSE 0 END,
[weekbeforelast] = CASE WHEN Grp = 'weekbeforelast' THEN Result ELSE 0 END
FROM Preaggregate
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden