how to give same serial number for group of records in mysql

Question

I am just new in mysql. I have records like below in carts table

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

M

Answer 1

Your groups are defined by starting with code = 100. This is a little tricky. In MySQL 8+, you can use window functions to define the groups and then row_number() to assign the 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;

This is much more complicated in earlier versions of MySQL.

You can identify the group using a subquery:

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

You can then use variables to get the information you want:

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;

Answer 2

According to your example you always have the same serial number for a code. So what do you gain?

Here is the very simple query:

select id, code, code - 99 as serial_number
from mytable
order by id;