如何使用JOIN在1对多关系中找到最新行（日期和时间）

Question

I have two tables: users( user_id , country) and message( time_date , currency, amount_sold, message_user_FK ). The tables are in a 1:Many relationship ie each user can have multiple messages. I would like to find the latest message for every country. I have the following query but it doesn't work. What am i doing wrong?

  SELECT u.country, s.date_time 
  FROM users u 
  JOIN message s ON u.user_id = s.message_user_FK
  JOIN(
  SELECT message_user_FK ,MAX(date_time) date_time 
  FROM message 
  GROUP BY message_user_FK 
  ) s1
  ON(s.message_user_FK = s1.message_user_FK  AND s.date_time =   s1.date_time );

http://sqlfiddle.com/#!9/03858/20

Answer 1

If you want the country with the latest date time, use a simple query . . . join and group by :

SELECT u.country, MAX(s.date_time)
FROM users u JOIN
     message m
     ON u.user_id = m.message_user_FK
GROUP BY u.country;

Answer 2

What's wrong with this?

SELECT u.country ,MAX(m.date_time) date_time 
FROM users u
     JOIN message m on u.user_id = m.message_user_FK
GROUP BY u.country

You subquery was close -- you were just grouping by the incorrect field. This would do the same.

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• Your updated fiddle