Calculate the avarage without non-zero values and given zero values in the next column.

avg_nullif.sql

with data as (
    select 50 as col1,  0 as col2 from dual union all
    select  0 as col1, 20 as col2 from dual union all
    select 20 as col1, 10 as col2 from dual union all
    select 20 as col1, 10 as col2 from dual
)
select avg(nullif(col1, 0)) as col1_avg_no_zero,
       avg(nullif(case when col2 = 0 then 0 else col1 end, 0)) as col1_avg_no_zero_with_col2        
from   data;