mysql - 显示包含最新编辑版本的所有记录

Question

我有一张包含学生姓名的表格，自动增加 NR 记录，但每次编辑记录时，都会创建一个新记录，将 NR 复制到 >ID 字段。 但是当我尝试在 MAX(NR) 时对 ID 记录进行分组时，它会显示该 ID 的最大数量，但是当我询问剩余的 rocord 时，它不会显示最后一条记录该组 ID 位于

SELECT MAX(`NR`) AS 'mNr',`NR`,`ID`,`Name1`,`Name3`,`Gender`
  FROM `Kids`  GROUP BY `ID`

这会产生如下结果:

mNr NR  ID  Name1   Name3   Gender
252 1   1   Alice   Carper  f
179 2   2   Dorah   Fisher  f
189 3   3   Racheal King    f
173 4   4   Frank   Smith   m
192 5   5   Patrick Fay m
305 6   6   Gloria  Sing    f
299 7   7   Bridget Young   f

But as you can see, the query shows the highest edit NR, but then continues to give the lowest of the rest of the record, not the record details belonging to that lastest NR... What am I doing wrong? This is the sample data:

NR  ID  Name1   Name3   Gender
1   1   Alice   Achand  f
2   2   Dorah   Achieng f
3   3   Racheal Achieng f
4   4   Francisca   Adikin  f
5   5   Patrick Adilu   m
6   6   Gloria  Ajwang  f
7   7   Bridget Aketch  f
130 5   Patrick Adilu   m
129 4   Francisca   Adikin  f
128 2   Dorah   Achieng f
153 4   Francisca   Adikin  f
173 4   Francisca   Adikin  f
179 2   Dorah   Achieng f
189 3   Racheal Achieng f
192 5   Patrick Adilu   m
252 1   Alice   Wor f
299 7   Bridget Aketch  f
305 6   Gloria  Ajwang  f

Answer 1

也许您不知道，您正在使用 MySQL 的一个神秘功能。 MySQL 允许您在聚合查询的 select 语句中包含不在聚合函数或 group by 子句中的列。引擎为这些列输入任意值。

做你想做的事情的正确方法是使用连接:

SELECT k.*
FROM `Kids` k join
     (select id, max(nr) as maxnr
      from kids
      group by id
     ) m
     on k.id = m.id and nr = maxnr;

以下是文档中的明确解释:

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. This means that the preceding query is legal in MySQL. You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values within each group the server chooses.

您可以更详细地阅读 here .