Sql 加入，計數引用包括零

Question

我制作了一個簡單的數據庫，其中包含兩個表，“國家”和“人類”：

+------+---------+
| id   | name    |
+------+---------+
|    1 | Finland |
|    2 | Sweden  |
|    3 | Norway  |
|    4 | Denmark |
+------+---------+

+--------+----------+
| name   | homeland |
+--------+----------+
| Liisa  |        1 |
| Matti  |        1 |
| Holger |        2 |
+--------+----------+

這個查詢表明沒有人住在挪威，但為什么它沒有提到丹麥？

select country.name, count(human.name)
from human
    right join country on human.homeland = country.id
group by homeland;

+---------+-------------------+
| name    | count(human.name) |
+---------+-------------------+
| Norway  |                 0 |
| Finland |                 2 |
| Sweden  |                 1 |
+---------+-------------------+

我正在使用 MariaDB，但更喜歡適用於其他數據庫的解決方案。

Answer 1

您通常GROUP BY與您相同的列SELECT ，除了那些 arguments 設置功能：

select country.name, count(human.name)
from country 
    left join human on human.homeland = country.id
group by country.name;