为什么SQL不存在返回所有空值的1记录

SQL Fiddle on the following:

SQL Fiddle:

create table tbl( col1 int, col2 int, col3 int);
insert into tbl values(1,1,1);
insert into tbl values(1,1,1);

select sum(col1) c1, sum(col2) c2, sum(col3)c3
from tbl
where not exists (
  select 2 as c1, 2 as c2, 2 as c3
  )
cr