在SQL如何从列中找到最大值

Question

假设在一个表中我有5列（a，b，c，d，e）每个整数值，并且这个表有100行，我们如何才能找到最大值（a），sum（b），sum（c），sum（d），sum（e））和列名

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; 

在数据库中没有此功能，您可以使用case或逆转置和reaggregate：

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。一些数据库使用TOP或LIMIT而不是FETCH FIRST。