Du kan gå med i den underfrågan så här:
update n1 set
isvalid = 0
from
ncaastats n1
inner join (
SELECT
FirstName, LastName, NCAAStats.AccountId, College_Translator.school_name, StatTypeId, COUNT(*) AS 'Count'
FROM NCAAstats
INNER JOIN College_Translator
ON College_Translator.AccountID = NCAAstats.AccountId
GROUP BY FirstName, LastName, NCAAStats.AccountId, College_Translator.school_name, CalendarYear, StatTypeId
HAVING COUNT(*) >1
) n2 on
n1.accountid = n2.accountid