简单的mysql连接无法正常工作

Question

我有

SELECT country.* , COUNT( country.coid ) AS city_count_for_country
FROM `air_countries` AS country
JOIN `air_cities` AS city ON city.coid = country.coid

但它只会返回一个国家/地区，city_count_for_country不会计算一个国家/地区的所有城市，而是所有城市。 怎么了？

谢谢

Answer 1

您想要的只有通过group by关键字才能实现； 单独的JOIN永远不会聚合行； 你可能想写

SELECT country.* , COUNT( country.coid ) AS city_count_for_country
   FROM air_countries AS country
   JOIN air_cities AS city
   ON city.coid = country.coid
   GROUP BY country.coid

Answer 2

您需要将其分组

SELECT country.* , COUNT( country.coid ) AS city_count_for_country
FROM `air_countries` AS country
JOIN `air_cities` AS city ON city.coid = country.coid
GROUP BY country.coid

Answer 3

通过添加到查询中，尝试使用分组功能

GROUP BY country.coid

Answer 4

您缺少GROUP BY子句。 尝试这个：

select country.coid, count(*) as city_count_for_country
from `air_countries` as country
join `air_cities` as city on city.coid = country.coid
group by country.coid;