Search with Multiple Choice problem

Fred

I use these Formula and work but the problem is I have only 3 items in the table Tipo de Documento and need to use aleatory case number. I try one by one I'm lucky to find the case number, and if I insert another Tipo de Documento have problems.

switch number(Busqueda) do
case 1:
    (select Documentos where 'Tipo Documentos'.Clase = "Plano" and 'Ganado REAL' = null)
case 4:
    (select Documentos where 'Tipo Documentos'.Clase = "Actividad" and 'Ganado REAL' = null)
case 8:
    (select Documentos where 'Tipo Documentos'.Clase = "Documento" and 'Ganado REAL' = null)
end