>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.