Did you include the changes to the Where clause that Sergey suggested?

FROM 
	holding thold 
	left outer join holding yhold on (thold.portid = yhold.portid and thold.securityid = yhold.securityid) 
	inner join security on thold.securityid = security.securityid 
	inner join portfolio on thold.portid = portfolio.portid 
WHERE 
	thold.date = @asofdate 
	and (yhold.date IS NULL OR yhold.date = @prevdate)

I can think of no reason that this wouldn't work.

-Mike