How to deal with null values while DIVIDING

Question

I have a simple looking problem but I don't know how to handle. I have tow columns which are filled by value or null.

I have to make an average of these like this:

1. if in both are values = (A+B)/2
2. if one is null then = A or B.

Is it possible to write it in different way then:

 case when a is not null and b is not null then....
 etc.

If I use a simple (a+b)/2 I get null in cases where one of values is null.

Answer 1

Probably the simplest way is to use outer apply with avg() because avg() ignores NULL values:

select v.avg_ab
from t outer apply
     (select avg(x) as avg_ab
      from (values (t.A), (t.B)
           ) v
     ) v;

You can also do this with a complicated case expression:

select (case when A is not NULL and B is not NULL then (A + B) / 2
             when A is not NULL then A
             when B is not NULL then B
        end) as avg_ab
. . .

This works well enough for 2 values; it is feasible for 3. It doesn't generalize well beyond that. Another way to use case is a bit more generalizable:

select ( (coalesce(A, 0) + coalesce(B, 0)) /
         ((case when A is not null then 1 else 0 end) +
          (case when B is not null then 1 else 0 end)
         )
       )

But the apply method is still simpler.