LEFT JOIN on Max Value

Question

假设我有以下两个表：

STUDENT
studentid   lastname   firstname
1           Smith      John
2           Drew       Nancy

STUDENT_STORY
studentid   dateline   storyid   status
1           1328313600 10        2
1           1328313601 9         1
2           1328313602 14        2
2           1328313603 12        1

现在，我需要一个SQL查询，在学生故事表中选择每个学生以及该学生的最新故事。

我在尝试这个：

SELECT s.*, ss.*
FROM student AS s
LEFT JOIN (
    SELECT *
    FROM student_story
    WHERE student_story.studentid = s.studentid
    ORDER BY dateline DESC LIMIT 1
) AS ss ON (ss.studentid = s.studentid)

但是，此查询不起作用。 它抱怨s.studentid是子查询的where子句中的未知字段。

请建议我如何实现我想要做的事情。

谢谢。

Answer 1

尝试这样的事情：

SELECT
  s.*,
  ss.*
FROM
  student AS s
LEFT JOIN
  student_story AS ss
ON (ss.studentid = s.studentid)
WHERE ss.dateline = (
  SELECT
    MAX(dateline)
  FROM
    student_story AS ss2
  WHERE
    ss2.studentid = s.studentid
)

Answer 2

SELECT s.*, ss.*
FROM student AS s
LEFT JOIN (

SELECT * FROM student_story    
ORDER BY dateline DESC LIMIT 1
) 
AS ss ON (ss.studentid = s.studentid)

Answer 3

SELECT 
    s.sale_id,
    s.created_at,
    p.created_at,
    DATEDIFF(p.created_at, s.created_at) AS days
FROM
    pos_sales s
        LEFT JOIN
    pos_payments p ON p.sale_id = s.sale_id
        AND
    p.created_at = (SELECT 
            MAX(p2.created_at)
        FROM
            pos_payments p2
        WHERE
            p2.sale_id = p.sale_id)

Answer 4

这个连接应该这样做：

SELECT s.*, ss.*
FROM student_story AS ss
LEFT JOIN student AS s
    ON student_story.studentid = s.userid
GROUP BY ss.studentid

GROUP BY应该占据最后一行，所以最后一个故事=最后一行，如果它不起作用，请尝试：

GROUP BY ss.studentid, ss.dateline DESC

我不确定，请对结果发表评论

Answer 5

您需要在student_story表的每一行上添加一个自动和唯一ID。

然后你会区分它们。 （假设studentstory.new_id）

select s.*, ss.*
from student s
left join student_story ss on ss.studentid = s.userid
where ss.new_id = ( select ss.new_id from student s
    group by dateline
    order by dateline desc
    limit 1
)

Answer 6

另一种方法是使用NOT EXISTS()条件进行LEFT JOIN 。 根据具体情况，这可以提供更多的灵活性; 但是，如果每个学生有两个具有相同dateline条目，它也会显示重复的结果：

SELECT s.*, ss.*
FROM student AS s
LEFT JOIN student_story AS ss ON ss.studentid = s.studentid AND NOT EXISTS
    (SELECT * FROM student_story AS ss2
     WHERE ss2.studentid = ss.studentid AND ss2.dateline > ss.dateline)