bagaimana cara memilih AVG dari beberapa kolom pada satu baris?

Coba

 Select     (Coalesce(x,0) + Coalesce(y,0) + Coalesce(z,0)) /
       (Coalesce(x/x, 0) + Coalesce(y/y, 0) + Coalesce(z/z, 0))

atau

 Select (Coalesce(x,0) + Coalesce(y,0) + Coalesce(z,0)) /
         (Case When x Is Null 0 Else 1 End +
          Case When y Is Null 0 Else 1 End +
          Case When z Is Null 0 Else 1 End)