psql，显示不在group by子句中的列

Question

我在查询时遇到问题。我有两个表：国家和城市，我想显示每个国家/地区人口最多的城市。

查询如下：

select country.name as coname, city.name as ciname, max(city.population) as pop 
    from city 
    join country on city.countrycode=country.code 
    group by country.name 
    order by pop;`

错误

column "city.name" must appear in the GROUP BY clause or be used in an aggregate function.

我不知道如何解决这个问题，我尝试进行子查询但没有成功。我怎样才能让它工作？

Answer 1

您可以使用rank函数轻松获取它：

select * from
(
select country.name as coname, 
 city.name as ciname, 
 city.population, 
 rank() over (partition by country.name order by city.population desc) as ranking 
from 
    city 
join 
    country 
on city.countrycode=country.code 
 ) A
 where ranking = 1

psql，显示不在group by子句中的列

psql, display column that is not in the group by clause

postgresql

join

aggregation