SQL 按 desc 排序不适用于 group by

Question

我试图用列 tahun 和 semester 获取最新的列 id_status（时间戳完全相同所以我不能使用它），我可以用这段代码没问题:

SELECT A.id_ips, B.tahun, B.semester, B.id_status FROM data_ips A
LEFT JOIN data_status_mahasiswa B
ON A.id_mahasiswa = B.id_mahasiswa
ORDER BY B.tahun DESC, B.semester DESC

id_ips	tahun	semester	id_status
155324	2012	2	6
155324	2012	1	7
155324	2009	2	1
155324	2009	1	2
155324	2008	2	9
155324	2008	1	1
155325	2015	2	8
155325	2015	1	3
155325	2010	2	4
155325	2010	1	9
155325	2009	2	9
155325	2009	1	4

但是当我尝试用 id_ips 列对其进行分组时，数据似乎不再是有序的...

SELECT X.* FROM 
(
    SELECT A.id_ips, B.tahun, B.semester, B.id_status FROM data_ips A
    LEFT JOIN data_status_mahasiswa B
    ON A.id_mahasiswa = B.id_mahasiswa
    ORDER BY B.tahun DESC, B.semester DESC
) AS X
GROUP BY X.id_ips

id_ips	tahun	semester	id_status
155324	2008	1	1
155325	2009	1	4

我试过像这样在同一个地方进行 GROUP BY 和 ORDER BY：

SELECT A.id_ips, B.tahun, B.semester, B.id_status FROM db_mahasiswa.data_ips A
LEFT JOIN db_mahasiswa.data_status_mahasiswa B
ON A.id_mahasiswa = B.id_mahasiswa
GROUP BY A.id_ips
ORDER BY B.tahun DESC, B.semester DESC

还有这个：

SELECT X.* FROM 
(
    SELECT A.id_ips, B.tahun, B.semester, B.id_status FROM data_ips A
    LEFT JOIN data_status_mahasiswa B
    ON A.id_mahasiswa = B.id_mahasiswa
    ORDER BY B.tahun DESC, B.semester DESC
) AS X
GROUP BY X.id_ips
ORDER BY X.tahun DESC, X.semester DESC

但是结果还是一样，我要的是：

id_ips	tahun	semester	id_status
155324	2012	2	6
155325	2015	2	8

不知道哪里出了问题...?

Answer 1

由于您想要每个 id_ips 的 tahun、学期和 id_status 的最高值，请尝试使用 group by 子句聚合 max()。

  SELECT X.id_ips,max(X.tahun) as tahun, max(X.semester) as semester, B.id_status  as id_status FROM 
    (
        SELECT A.id_ips, B.tahun, B.semester, B.id_status,id_mahasiswa FROM data_ips A
        LEFT JOIN data_status_mahasiswa B
        ON A.id_mahasiswa = B.id_mahasiswa
        ORDER BY B.tahun DESC, B.semester DESC
    ) AS X
    left jion data_status_mahasiswa B
    on X.id_mahasiswa = B.id_mahasiswa 

    GROUP BY X.id_ips
    ORDER BY X.tahun

Answer 2

这个查询：

SELECT X.*
FROM (SELECT A.id_ips, B.tahun, B.semester, B.id_status
      FROM data_ips A LEFT JOIN 
           data_status_mahasiswa B
           ON A.id_mahasiswa = B.id_mahasiswa
      ORDER BY B.tahun DESC, B.semester DESC
     ) X
GROUP BY X.id_ips;

根本不正确，应该会产生语法错误。为什么？外部查询中的 GROUP BY 列与 SELECT 列不一致。令人高兴的是，这 returns 是几乎所有数据库中的错误。

如果这是你想要的：

I tried to get the latest column id_status with column tahun and semester (the timestamp is all the same so I can't use it)

然后一种方法使用 window 函数：

SELECT X.*
FROM (SELECT A.id_ips, B.tahun, B.semester, B.id_status,
             ROW_NUMBER() OVER (PARTITION BY A.id_ips ORDER BY B.tahun DESC, B.semester DESC) as seqnum
      FROM data_ips A LEFT JOIN 
           data_status_mahasiswa B
           ON A.id_mahasiswa = B.id_mahasiswa
     ) X
WHERE seqnum = 1;

请注意，此查询中根本没有聚合。你不是在“聚合”任何东西。您只想选择正确的行，这是通过 filtering 而不是 aggregation.

Answer 3

您想要的结果的解决方案是不使用 group by，因为您没有聚合任何东西，您只是根据两个后续列的顺序为每个 id_ips 选择一行， row_number 最适合：

with cte as (
    select *, Row_Number() over(partition by id_ips order by tahun desc, semester desc) rn
    from t
)
select id_ips, tahun, semester, id_status
from cte
where rn=1

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