sql - 数据库查询 : How to count maximum for several columns

Question

假设我有下表

claim_date   person_type
------------------------
01-01-2012         adult
05-05-2012         adult
12-12-2012         adult
12-12-2012         adult
05-05-2012         child
05-05-2012         child
12-12-2012         child

当我执行以下查询时:

select 
    claim_date, 
    sum(case when person_type = 'adult' then 1 else 0 end) as "nbr_of_adults",
    sum(case when person_type = 'child' then 1 else 0 end) as "nbr_of_children"
  from my_table
group by claim_date
;

我在这里得到这个结果:

claim_date   nbr_of_adults    nbr_of_children
---------------------------------------------
01-01-2012               1                  0
05-05-2012               1                  2
12-12-2012               2                  1

我想收到的是最大数量的成人(此处:2)和最大数量的 child (此处:2)。
有没有办法通过单个查询来实现这一目标？感谢您的任何提示。

Answer 1

使用派生表获取计数，然后选择最大值:

select max(nbr_of_adults) max_adults,
       max(nbr_of_children) max_children
from
(
  select 
      sum(case when person_type = 'adult' then 1 else 0 end) as "nbr_of_adults",
      sum(case when person_type = 'child' then 1 else 0 end) as "nbr_of_children"
    from my_table
  group by claim_date
) a