MySQL - Counter within group

Question

I would like to add a counter for each row within a group according an ascending variable. I have a solution but it does not work if some variable within groups are equal:

Answer 1

User Defined Variables become handy when solving this issues. This should work:

select g, x, counter from (
    select g, x,
        @counter := if (g = @prev_g, @counter + 1, 1) counter,
        @prev_g := g
    from tb, (select @counter := 0, @prev_g := null) init
    order by g, x
) s

If you don't really mind the fourth column then you could safely remove the outer select. That would improve performance a lot.