Multi-base conversion - using all combinations for URL shortener

I am making an URL shortener, and I am struggling with the optimal way of encoding a number (id) into a character string.

I am using the characters 0-9,A-Z,a-z so it will basically be a base-62 encoding. That is pretty basic, but it doesn't make use of all possible codes. The codes that it would produce would be:

0, 1, ... y, z, 10, 11, ... zy, zz, 100, 101, ...

Notice that the codes 00 to 0z is not used, the same for 000 to 0zz, and so on. I would like to use all the codes, like this:

0, 1, ... y, z, 00, 01, ... zy, zz, 000, 001, ...

It would be some combination of base-62 and base-63, with different bases depending on the position... Using base-62 is easy, for example:

create procedure tiny_GetCode
    @UrlId int
as
set nocount on

declare @Code varchar(10)
set @Code = ''

while (@UrlId > 0 or len(@Code) = 0) begin
    set @Code = substring('0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz', @UrlId % 62 + 1, 1) + @Code
    set @UrlId = @UrlId / 62
end

select @Code

But I haven't yet managed to make a multi-base conversion out of it, to make use of all the codes.

I managed to make the conversion. The tricky thing is that it's not just a mixed base conversion, the higher base of the first character also affects the values of longer codes.

I started with an easier case; base-10 codes. I saw that the two digit range has 10 extra codes, the three digit range has 100 extra codes, and so on:

0 - 9        : '0' - '9'
10 - 109     : '00' - '99'
110 - 1109   : '000' - '999'
1110 - 11109 : '0000' - '9999'

So, the value of the first character in the code is not just the base raised to the position, but it also has an offset.

After applying this to the base-62 encoding, this is what I ended up with:

create function tiny_Encode(@UrlId int) returns varchar(10)
as
begin

  declare
    @Chars varchar(62),
    @Code varchar(10),
    @Value int,
    @Adder int

  set @Chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
  if (@UrlId < 63) begin
    set @Code = substring(@Chars, @UrlId, 1)
  end else begin
    set @UrlId = @UrlId - 1
    set @Value = 62
    set @Adder = 0
    while (@UrlId >= @Value * 63 + @Adder) begin
      set @Adder = @Adder + @Value
      set @Value = @Value * 62
    end
    set @Code = substring(@Chars, (@UrlId - @Adder) / @Value, 1)
    set @UrlId = ((@UrlId - @Adder) % @Value)
    while (@Value > 1) begin
      set @Value = @Value / 62
      set @Code = @Code + substring(@Chars, @UrlId / @Value + 1, 1)
      set @UrlId = @UrlId % @Value
    end
  end
  return @Code

end