SELECT 
  IIf(DateDiff("s",Schede.[data Scheda],Schede.scad)>0,IIf(DateDiff("s",Schede.[data Scheda],Schede.scadc)>0,Schede.[data Scheda],Schede.scadc),IIf(DateDiff("s",Schede.scad,Schede.scadc)>0,Schede.scad,Schede.scadc)) AS DataMinore
FROM 
  Schede;