Update with a Join, Group By, and Having

The select statement executes with no errors or warning.

The update statement throws an error:
Incorrect syntax near the keyword 'group'.

select [sSVsys].[textUniqueWordCount], count(*) as [actCount] 
from [docSVsys]  as [sSVsys]with (nolock)
join [FTSindexWordOnce] with (nolock)
  on [sSVsys].[sID] = [FTSindexWordOnce].[sID]
where [sSVsys].[sID] < 500000
group by [sSVsys].[sID], [sSVsys].[textUniqueWordCount] 
having [sSVsys].[textUniqueWordCount] <> count(*)

update [sSVsys]
set [sSVsys].[textUniqueWordCount] = count(*) 
from [docSVsys]  as [sSVsys]with (nolock)
join [FTSindexWordOnce] with (nolock)
  on [sSVsys].[sID] = [FTSindexWordOnce].[sID]
where [sSVsys].[sID] < 500000
group by [sSVsys].[sID], [sSVsys].[textUniqueWordCount] 
having [sSVsys].[textUniqueWordCount] <> count(*)

If the answer is to join to a derived table then I can figure that out.
Do I have a syntax error with the existing update?

This derived table worked

update [docSVsys] 
set [docSVsys].[textUniqueWordCount] = [WordOnce].[actCount]
from [docSVsys]
join 
(   select [FTSindexWordOnce].[sID], count(*) as [actCount]
    from   [FTSindexWordOnce] with (nolock)
    -- where  [FTSindexWordOnce].[sID] < 1500000
    group by [FTSindexWordOnce].[sID]  ) as [WordOnce]
 on [docSVsys].[sID] = [WordOnce].[sID]
and [docSVsys].[textUniqueWordCount] <> [WordOnce].[actCount]

I will leave this up for a couple days for any comments or answers on a better approach and then just delete. This approach is in an existing SO answer.

Your original update statement has GROUP BY and HAVING, which are not allowed in the UPDATE statement syntax. Here's a link to a syntax diagram: UPDATE (Transact-SQL).

Your second version has the GROUP BY and HAVING as part of a derived table, which is allowed.

So, yeah: you did have a syntax error.

Incidentally, I agree with @bluefeet: a CTE in place of a derived table would make your update easier to read and understand. A small thing, but it can make a big difference in ease of maintenance.