>>Just to fully understand your requirements: do you want to retrieve FirstPerson and LastPerson for each one of Client.Numero or just for an specific one that is set somewhere else in your application?
>
>This is to extract the first occurence of the field and the last occurence of the field as per one specific master record. So, if a specific master record has 5 child records, the subset will be done from those 5 records. And, to determine which one is subject to be selected, the order is based on AddDate. That's why the second one is DESC.

Ok, I think I got it (hopefully). A simple query with no subqueries:

select top 1 i1.ApprovedPersonName as FirstPerson, i2.ApprovedPersonName as LastPerson
	from Invoice i1 inner join Invoice i2 on i2.NoClient = i1.NoClient
	where i1.NoClient = 1
	order by i1.AddDate, i2.AddDate desc

will give you back Michel Fournier and King Kong (I remember the days you were an extra in a movie, glad to see you now as a main character!).