如何为mysql中的一组记录提供相同的序列号

我是 mysql 的新手。我在购物车表中有如下记录

 id   code
 1     100
 2     101
 3     102
 4     100
 5     100
 6     101

我的例外输出如下：-

id     code    serial_number
 1     100        1
 2     101        2
 3     102        3
 4     100        1
 5     100        1
 6     101        2

我想要属于同一组的相同序列号，即代码。谁能帮我怎么办？

您的组的定义是从code = 100。这有点棘手。在 MySQL 8+ 中，您可以使用窗口函数来定义组，然后row_number()来分配serial_number:

select c.*, 
       row_number() over (partition by grp order by id) as serial_number
from (select c.*,
             sum( code = 100 ) over (order by id) as grp
      from carts c
     ) c;

这在 MySQL 的早期版本中要复杂得多。

您可以使用子查询来识别组：

select c.*,
       (select count(*)
        from carts c2
        where c2.id <= c.id and c2.code = 100
       ) as grp
from carts c;

然后您可以使用变量来获取您想要的信息：

select c.*,
       (@rn := if(@g = grp, @rn + 1,
                  if(@g := grp, 1, 1)
                 )
       ) as serial_number
from (select c.*,
             (select count(*)
              from carts c2
              where c2.id <= c.id and c2.code = 100
             ) as grp
      from carts c
      order by grp, id
     ) c cross join
     (select @g := -1, @rn := 0) params;