在sql中如何从列中找到max

Question

假设在一个表中我有5列（a，b，c，d，e）每个都是一个整数值，这个表有100行，我们如何从每列的总和和列名中找到最大值sql？即max（sum（a），sum（b），sum（c），sum（d），sum（e））和columnname

Answer 1

在许多数据库中，您可以使用greatest()：

select greatest(sum(a), sum(b), sum(c), sum(d), sum(e))
from t;

在没有此功能的数据库中，您可以使用case或取消忽略并重新聚合：

select sum(val)
from ((select a as val, 'a' as which from t) union all
      (select b as val, 'b' as which from t) union all
      (select c as val, 'c' as which from t) union all
      (select d as val, 'd' as which from t) union all
      (select e as val, 'e' as which from t)
     ) abcde
group by which
order by sum(val) desc
fetch first 1 row only;

这是ANSI语法。有些数据库使用TOP或LIMIT代替FETCH FIRST。