Question

我有以下表格：

tasks ( id int PRIMARY KEY auto_increment, 
        description text, 
        created datetime, 
        modified datetime);

commentaries( id int PRIMARY KEY auto_increment, 
              task_id int, 
              user_id int, 
              description text,
              created datetime );

users ( id int PRIMARY KEY auto_increment,
        name varchar(300),
        created datetime );

这是一对多关系（1-N）。

我正在构建一个收件箱，列出最后N个任务，并显示每个任务的最后插入的评论？

EDIT1 ：

查询：

SELECT T.id, T.description, C.description, T.created
FROM tasks T LEFT JOIN commentaries C ON T.id = C.task_id AND C.id IN (
                SELECT max(C.id) FROM tasks Task
                LEFT JOIN commentaries C ON C.task_id = Task.id
                WHERE Task.id in (select id from tasks WHERE user_id = 1 )
                GROUP BY Task.id)
ORDER BY T.id DESC
LIMIT 10;

EDIT2

SQL小提琴链接：http://sqlfiddle.com/#!2/1555c7/1

Answer 1

您可以使用以下方式获取上次插入评论的时间：

select task_id, max(created) as maxc
from commentaries
group by task_id;

您可以在join中使用此评论来获取最后插入的评论：

select t.*, c.*
from tasks t join
     commentaries c
     on c.task_id = t.id join
     (select task_id, max(created) as maxc
      from commentaries
      group by task_id
     ) cmax
     on cmax.task_id = c.task_id and cmax.maxc = c.created;

Answer 2

SELECT T.*, C.*
FROM tasks T
     JOIN Commentaries C
          ON T.id = C. task_id
GROUP BY T.id
ORDER BY C.task_id ASC
LIMIT 1

Answer 3

select * from task,com   where task.id=com.task_id  order by com.da desc limit 1;

MYSQL 1到N JOIN

3 个答案: