mask

 select
     count(1) as cnt
 from (
     select '' as flag union all
     select 'a' as flag union all
     select null as flag
 ) t0
 where flag <> 0
 ;

 select
     count(1) as cnt
 from (
     select '' as flag union all
     select 'a' as flag union all
     select null as flag
 ) t0
 where flag <> ''
 ;

 select
     count(1) as cnt
 from (
     select '' as flag union all
     select 'a' as flag union all
     select null as flag
 ) t0
 where nvl(flag,'') <> ''
 ;