如何通过加入的 hstore 键对组进行区分？

Question

我正在尝试使用 postgres 执行以下操作：

• 计数不同
• 表连接
• 按 hstore 键分组

我不认为我太过分了，但是不同的计数不是按值按组累加的。

Here is the code on rextester.com

到目前为止我所拥有的：

SELECT COUNT(DISTINCT pets.id),locations.attr -> 'country' as country
FROM pets,photos,locations
WHERE photos.pet_id = pets.id
AND photos.location_id = locations.id
GROUP BY pets.id,locations.attr -> 'country';

这给了我：

而我想要：

Answer 1

从GROUP BY 中丢失pets.id：

SELECT COUNT(DISTINCT pets.id),locations.attr -> 'country' as country
FROM pets,photos,locations
WHERE photos.pet_id = pets.id
AND photos.location_id = locations.id
GROUP BY locations.attr -> 'country';

编辑：

您实际上并不需要加入宠物表。另外，使用显式 JOIN 语法：

select
    l.attr -> 'country' country,
    count(distinct p.pet_id)
from photos p
inner join locations l
on p.location_id = l.id
group by l.attr -> 'country';

不使用COUNT(DISTINCT)：

select 
    country, count (pet_id)
from (
    select
        l.attr -> 'country' country,
        p.pet_id
    from photos p
    inner join locations l
    on p.location_id = l.id
    group by l.attr -> 'country', p.pet_id
) t
group by country;

http://rextester.com/YVR16306