postgresql中group by的嵌套json聚合行

Question

我有一个 postgresql 表

alok=# select * from people;
  name   | country |  city   
---------+---------+---------
 alok    | india   | delhi
 ram     | india   | kolkata
 shyam   | india   | kanpur
 krishna | us      | newyork
 ajit    | india   | delhi
 sami    | india   | delhi
 mohamad | india   | kolkata
(7 rows)

可以由

产生

create table people(name varchar, country varchar, city varchar);

insert into people values('alok', 'india', 'delhi');
insert into people values('ram', 'india', 'kolkata');
insert into people values('shyam', 'india', 'kanpur');
insert into people values('krishna', 'us', 'newyork');
insert into people values('ajit', 'india', 'delhi');
insert into people values('sami', 'india', 'delhi');
insert into people values('mohamad', 'india', 'kolkata');

我想生成国家和城市的聚合名称，例如

country |                name                
---------+------------------------------------
 india   | {"delhi":["alok", "ajit", "sami"], "kolkata":["ram", "mohamad"], "kanpur":["shyam"]}
 us      | {"newyork":["krishna"]}

Answer 1

demo:db<>fiddle

SELECT 
    country,
    json_object_agg(city, agg_names)
FROM (
    SELECT
        country,
        city, 
        json_agg(name) AS agg_names
    FROM
        people
    GROUP BY country, city
) s
GROUP BY country

1. 按city 对名称进行分组。要将 names 放入一个 JSON 数组中，您可以使用聚合函数 json_agg()
2. 将此结果按country 分组。要获得预期的输出，您需要使用聚合函数 json_object_agg() 从 city 和 name 数组创建键/对值

documentation

Answer 2

select 
   country, json_agg( json_build_object( 'city', city, 'name', name  ) ) 
from (
   select country, city, json_agg( name ) as name
   from people
   group by country, city 
) a 
group by a.country;