如下图， 计划实现 ：按照 parent_code 分组， 取组中code最大值所在的整条记录，如红色部分。（类似hive中： row_number() over(partition by)）

select c.* from(   select a.*, (@i := case when @key_i=parent_code then @i+1 else 1 end) as sort_num,(@key_i:=parent_code) as tmp         from my_test a, (SELECT  @i := 0,@key_i:='') b        order by parent_code,code desc) c where c.sort_num=1;

个人理解， mysql 运行顺序：  from >>  where >> order by >> select .

 

 

以下部门应该有助于理解mysql中变量的概念。

1.=========================================================================================================

仅按照code排序,加序号sort_num效果如下：  

select (@i := @i + 1) as sort_num, a.*      from my_test a, (SELECT  @i := 0) b    order by code;

2.=========================================================================================================

仅按parent_code分组，按照code排序，加序号.

select a.*, (@i := case when @key_i=parent_code then @i + 1 else 1 end) as sort_num,(@key_i:=parent_code) as tmp     from my_test a, (SELECT  @i := 0,@key_i:='') b    order by parent_code,code desc;

 

 

参考链接 ：https://blog.csdn.net/zzhhoubin/article/details/79839610

https://www.cnblogs.com/CharlieLau/p/6737243.html

https://blog.csdn.net/u011728683/article/details/78594070