>>I would rewrite it into a form like
>>
>>

>>UPDATE Client
>>	SET DayBetweenFirstAcceptSoldDate = CASE WHEN ....WHEN ... WHEN ... THEN ... ELSE 0 END
>>	FROM Client
>>		LEFT JOIN BusinessTime ON ....
>>		LEFT JOIN Company ON ....
>>		LEFT JOIN Research ON ....
>>

>>
>>This will give the optimizer a chance to get you the best performance possible.
>
>Yes, I had that at first but this creates a too long command as I need to repeat pretty much the entire base command. By putting the CASE WHEN inside the WHERE clause, this allows to keep the core of the command as is but only to add an exception down below.


No you do not have to do that...


Consider the following. By no means error free, as I cannot test, but to give you an idea.

>>

UPDATE Client
	SET DayBetweenFirstAcceptSoldDate = ISNULL(Seconds,0)
	FROM Client
		LEFT JOIN (SELECT SUM(CASE WHEN ... THEN ... ELSE ... END  ) as seconds, ClientID as clientid2
                                 FROM Client 
					LEFT JOIN Company ON Company.NoCountry=1 AND Company.Numero=Client.NoCompany
					LEFT JOIN Research ON Research.NoCountry=1 AND Research.Numero=Client.NoResearch
					LEFT JOIN BusinessTime ON BusinessTime.NoProvince = COALESCE(Company.NoProvince, Research.NoProvince, 84) AND 
											BusinessTime.NoHoliday=0 AND
											BusinessTime.Start<Client.QueryDate AND
											BusinessTime.[End]>=Client.FirstAcceptDate
				WHERE 	Client.FirstAcceptDate>'1899-12-30 00:00:00.000' AND
						Client.QueryDate>'1899-12-30 00:00:00.000' AND
						Client.QueryDate>=Client.FirstAcceptDate AND
						(Client.NoCompany>0 OR Client.NoResearch>0)
				GROUP BY ClientID)  as Temp ON ClientId = ClientID2

You can run the subquery as a separate SELECT to check whether it produces the right results