I don't understand you. Can you post DDL+ sample data + expected result?

Did you mean ROWS not FIELDS?

Your table design is very questionable but assuming no NULLs and no
negatives, try this:

SELECT
(f1 + f2 + f3 + f4 + f5 + f6 + f7 + f8 + f9
 + f10 + f11 + f12 + f13 + f14 + f15)
/
(SIGN(f1) + SIGN(f2) + SIGN(f3) + SIGN(f4) + SIGN(f5)
+ SIGN(f6) + SIGN(f7) + SIGN(f8) + SIGN(f9) + SIGN(f10)
+ SIGN(f11) + SIGN(f12) + SIGN(f13) + SIGN(f14) + SIGN(f15))
FROM Table1 ;

If you have to cope with negatives then change SIGN(x) to ABS(SIGN(x)).

In general for  conditional execution take a look at the CASE
expression.

you could use case

select (f1 + f2 + f3 + f4) / case when f1 = 0 and f2 = 0 and f3 = 0 and f4 =
0 then 1 else case when f1 = 0 then 0 else 1 end + case when f2 = 0 then 0
else 1 end + case when f3 = 0 then 0 else 1 end + case when f4 = 0 then 0
else 1 end end from tableName

Here are a couple of options:

select id,
 (f1*s1 + f2*s2 + f3*s3 + ... + f15*s15)
   / (s1 + s2 + s3 + ... + s15) as avgpos
from (select *,
       1+sign(sign(f1) -1) as s1,
       1+sign(sign(f2) -1) as s2,
       1+sign(sign(f3) -1) as s3,
       ...
       1+sign(sign(f15)-1) as s15
     from (select id,
             isnull(f1, 0) as f1,
             isnull(f2, 0) as f2,
             isnull(f3, 0) as f3,
             ...
             isnull(f15, 0) as f15
           from t1) as d1) as d2

select id, avg(val) as avgpos
from (select id, n,
       case n
         when 1 then f1
         when 2 then f2
         when 3 then f3
         ...
         when 15 then f15
       end as val
     from t1,
       (select 1 as n
        union all select 2
        union all select 3
        ...
        union all select 15) as nums) as d
where val > 0
group by id