MySql查询排序

Question

这是我的数据。我想要6行，但我希望所有HeadlineCategoryId在我的结果列表中都是唯一的。如果我选择前6名，我会从HeadlineCategoryID 20（6,2）中选择2行。你有什么建议吗？

Answer 1

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT  HeadlineCategoryID, MAX(Creation) max_date
            FROM    TableName
            GROUP   BY HeadlineCategoryID
        ) b ON  a.HeadlineCategoryID = b.HeadlineCategoryID AND
                a.Creation = b.max_date
ORDER   BY a.Creation DESC  -- << specify here how are you going to sort
LIMIT   6                   --    the records you want to get 

---

更新1

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT  HeadlineCategoryID, MAX(NewsID) max_id
            FROM    TableName
            GROUP   BY HeadlineCategoryID
        ) b ON  a.HeadlineCategoryID = b.HeadlineCategoryID AND
                a.NewsID = b.max_id
ORDER   BY a.Creation DESC  -- << specify here how are you going to sort
LIMIT   6                   --    the records you want to get 

Answer 2

看起来你想要六个最近的记录，但HeadlineCategoryId是唯一的。如果是这样，这将有效：

select top 6 NewsId, Creation, HeadlineCategoryId
from (select t.*,
             row_number() over (partition by HeadlineCategoryId order by Creation desc) as seqnum
      from t
     ) t
where seqnum = 1

作为一个说明。 。 。这个问题最初表明它使用的是SQL Server，而不是MySQL。 MySQL中的解决方案并不那么简单。以下是not exists的一种方法：

select NewsId, Creation, HeadlineCategoryId
from t
where not exists (select 1
                  from t t2
                  where t2.HeadlineCategoryId = t.HeadlineCategoryId and
                        t2.id < t.id)
limit 6

not exists部分说“没有其他记录在给定标题类别中有更大的ID”。