很可能，你应该重新评估你的db设计，无论如何，你可以尝试使用所有COL之间的差减去最后的2

select col1 + col2 + ... +col18 - t3.min2
from my_table 
cross join (
    select sum(my_col)*(-1) min2
    from (
        select  my_col
        from  (
            select col1 my_col cfrom  my_table
            union all 
            select col2 from  my_table
            union all 
            select col3 from  my_table
            ....
            union all 
            select col18 from  my_table

            ) t  
            order by my_col desc limit 2

    ) t2
) t3