как да избера AVG от множество колони на един ред

Опитайте

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

или

 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)