在SQL如何從列中找到最大值

Question

假設在一個表中我有5列（a，b，c，d，e）每個整數值，並且這個表有100行，我們如何才能找到最大值（a），sum（b），sum（c），sum（d），sum（e））和列名

Answer 1

在許多數據庫中，您可以使用greatest()：

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

在數據庫中沒有此功能，您可以使用case或逆轉置和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; 

這是ANSI語法。一些數據庫使用TOP或LIMIT而不是FETCH FIRST。