mysql - SQL Inner Join 语句未给出所需结果

Question

希望有人可以帮助我。我有以下两个表:

Table: locations
location_id   user_id       city        state
1             1             Los Angeles CA  
2             1             New York    NY
3             1             Chicago     IL
4             2             Dallas      TX
5             3             Denver      CO
6             4             Miami       FL
7             5             Atlanta     GA

Table: events
event_id    user_id     event_name    event_date
1           1           My Event 1    2017-02-01
2           2           My Event 2    2017-03-01
3           3           My Event 3    2017-04-01
4           4           My Event 4    2017-05-01
5           5           My Event 5    2017-06-01

我正在运行以下查询:

SELECT e.event_id,  e.user_id, e.event_name, e.event_date,
       l.user_id, l.city, l.state
FROM events e
INNER JOIN locations l
ON e.user_id = l.user_id
ORDER BY e.event_date ASC

我试图只获取事件表中的记录，但也拉取与两个表共有的 user_id 相匹配的相应城市和州。输出应该是:

 event_id   user_id     event_name    event_date    city          state
 1          1           My Event 1    2017-02-01    Los Angeles   CA
 2          2           My Event 2    2017-03-01    Dallas        TX
 3          3           My Event 3    2017-04-01    Denver        CO
 4          4           My Event 4    2017-05-01    Miami         FL
 5          5           My Event 5    2017-06-01    Atlanta       GA

谁能指出我在 SQL 语句中的错误吗？

Answer 1

您从未向我们提供用于决定为给定用户选择哪个位置的逻辑。一种方法是获取与给定用户关联的最小 location_id:

SELECT t1.*,
       COALESCE(t2.city, 'NA'),
       COALESCE(t2.state, 'NA')
FROM events t1
LEFT JOIN locations t2
    ON t1.user_id = t2.user_id
INNER JOIN
(
    SELECT user_id, MIN(location_id) AS min_location_id
    FROM locations
    GROUP BY user_id
) t3
    ON t2.user_id = t3.user_id AND
       t2.location_id = t3.min_location_id