sql - 数据库查询：如何计算多列的最大值

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

我想收到的是最大成人人数（这里：2）和最大儿童人数（这里：2）。有没有办法通过单个查询来实现这一点？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

Answer 2

使用嵌套查询：

    select max(nbr_of_adults) maxAd, max(nbr_of_children), maxCh from
    (
        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    
    )

Answer 3

我不知道你的 dbms 是什么，但在 sybase 上它可以工作：

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

Answer 4

select     
person_type,     
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 ;

Answer 5

如果您的 SQL 产品支持窗口聚合函数，您可以尝试如下操作：

SELECT DISTINCT
  MAX(COUNT(CASE person_type WHEN 'adult' THEN 1 END)) OVER () AS max_adult_count,
  MAX(COUNT(CASE person_type WHEN 'child' THEN 1 END)) OVER () AS max_child_count
FROM claim_table
GROUP BY claim_date

我还用条件 COUNT 替换了您的条件 SUM，这在我看来更清晰、更简洁。