>>I suggest to re-write this statement.
>>
>>1. Start from a select statement to return what you will need.
>>
>>2. Once you get your select statement working properly, turn it into MERGE command.
>
>The original select was what I needed. I only needed to add a condition in the WHERE clause to use one way or another based on the fact that if there is a company or not. The default, is to use the province from the Company table, if we have a Client.NoCompany. Otherwise, we use the province from the Research table, if we have Client.NoCompany=0 and a Client.NoResearch>0.

Start from this query:

;with cte as (select Client.*, COALESCE(Company.NoProvince, 84) as NoProvince 
from Client LEFT JOIN Company ON Client.NoCompany = Company.Numero and Company.NoCountry = 1
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
UNION ALL
select Client.*, COALESCE(Research.NoProvince, 84) as NoProvince 
from Client LEFT JOIN Research ON Client.NoReseach = Research.Numero and Research.NoCountry = 1
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.NoResearch>0)

select * from cte

This is what you're going to join with the sum of the BusinessTime.