Question

我有这个db：

用户表：

+----+---------+
| id | city_id |
+----+---------+

city_id - ＆gt;引用城市上的ID

城市表：

+----+------+
| id | city |
+----+------+

解答表：

+----+---------+
| id | user_id |
+----+---------+

user_id - ＆gt;引用用户ID

现在我想要一个包含此结果的查询：

+-----------+--------+
| City-name | amount |
+-----------+--------+
| New York  | 500    |
+-----------+--------+

我尝试了几次连接，计数，直接但没有成功。认为它应该很容易，但没有设法做到。

我的查询：

SELECT cities.name, COUNT(answers.id) FROM answers
JOIN users ON users.id = answers.user_id
JOIN cities on users.city_id = cities.id

但只给我一个城市..

Answer 1

您需要添加GROUP BY cities.name

SELECT cities.name, COUNT(answers.id) FROM answers
JOIN users ON users.id = answers.user_id
JOIN cities on users.city_id = cities.id
GROUP BY cities.name

这将告诉MySQL您希望COUNT按城市分组。如果没有这个，mysql会将所有内容组合在一起并统计一切。

Answer 2

你应该聚合以获得一个计数等等。所以像这样：

SELECT      COALESCE(c.city, '<Unknown>') AS Cityname,
            COUNT(a.id) AS amount

FROM        Answers AS a

LEFT JOIN   Users AS u
    ON      a.user_id = u.id

LEFT JOIN   Cities AS c
    ON      u.city_id = c.id

GROUP BY    c.city

获取每个城市的答案列表

2 个答案: