简单的mysql连接不能正常工作

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

但它只返回一个国家/地区而city_count_for_country不计算一个国家/地区的所有城市，而只计算所有城市。怎么了？

由于

Answer 1

您想要的只能通过group by关键字实现;仅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

尝试使用群组功能，添加到您的查询

GROUP BY country.coid

Answer 4

你错过了GROUP BY条款。试试这个：

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;