Update but only non-existing records

Hi,

I am trying to update a subtable but I want that, if it already certain certain records, create only new ones.

With the code below if certain records exists, it is duplicating the records.

let s := this;
for p in select 'Estudiantes Registrados' where Grupo = s.Grupo do
let a := (create Asistencia);
a.(Sesiones := s);
a.(Estudiante := p.Estudiantes);
a.('¿Asistió?' := "")
end

How can I update only non-existing records?

Thanks.

Jaime