Postgresql按列获得最大行数

Question

我正在尝试从表中每日计数总和中获取最大行数。我看过几篇看起来很相似的帖子，但似乎没有用。我试图按照 Get MAX row for GROUP in MySQL 但它不起作用在Postgres。下面是我有

select source, SUM(steps) as daily_steps, to_char("endTime"::date, 'MM/DD/YYYY') as step_date 
from activities 
where user_id = 1 
and "endTime" <= CURRENT_TIMESTAMP + INTERVAL '1 day' 
and "endTime" >= CURRENT_TIMESTAMP - INTERVAL '7 days' 
group by source, to_char("endTime"::date, 'MM/DD/YYYY') 

这将返回以下

source, daily_steps, step_date 
"walking";750;"11/17/2015" 
"walking";821;"11/22/2015" 
"walking";106;"11/20/2015" 
"running";234;"11/21/2015" 
"running";600;"11/24/2015" 

我想返回的结果只有那些由源daily_steps最大值的行。结果应该看起来像

source, daily_steps, step_date 
"walking";821;"11/22/2015" 
"running";600;"11/24/2015" 

Answer 1

的Postgres提供方便distinct on语法：

select distinct on (a.source) a.* 
from (select source, SUM(steps) as daily_steps, to_char("endTime"::date, 'MM/DD/YYYY') as step_date 
     from activities a 
     where user_id = 1 and 
      "endTime" <= CURRENT_TIMESTAMP + INTERVAL '1 day' and 
      "endTime" >= CURRENT_TIMESTAMP - INTERVAL '7 days' 
     group by source, to_char("endTime"::date, 'MM/DD/YYYY') 
    ) a 
order by a.source, daily_steps desc;