[英]LEFT JOIN on Max Value
Suppose I have the following two tables: 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
Now, I need an SQL query that would select each student along with the latest story for that student in the student story table. 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。
I am trying this: 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)
However, this query does not work. 但是,此查询不起作用。 It complains about s.studentid being an unknown field in the where clause of the sub query. 它抱怨s.studentid是子查询的where子句中的未知字段。
Please suggest how I can achieve what I'm trying to do. 请建议我如何实现我想要做的事情。
Thanks. 谢谢。
Try something like this: 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
)
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)
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)
This join should do it: 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
should take last row, so last story = last row, if it won't work try to: 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
I'm not sure about it, please comment with result 我不确定,请对结果发表评论
You'll need to add an auto and unique id on each row on student_story table. 您需要在student_story表的每一行上添加一个自动和唯一ID。
then you will distinguish between them. 然后你会区分它们。 ( assume studentstory.new_id ) (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
)
Another approach is making a LEFT JOIN
with a NOT EXISTS()
condition. 另一种方法是使用NOT EXISTS()
条件进行LEFT JOIN
。 Depending on the scenario, this can give a bit more flexibility; 根据具体情况,这可以提供更多的灵活性; however, it will also show duplicate results if there are two entries with the same dateline
per student: 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)
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