SQL: Is there a possibility to convert numbers (1,2,3,4…) to letters (A,B,C,D…)

Question

Is there a possibility to obtain letters (like A,B) instead of numbers (1,2) e.g. as a result of Dense_Rank function call(in MS Sql) ?

Answer 1

You can convert the values to an offset base-26 with a UDF:

EDIT: Corrected function.

create function dbo.HinkyBase26( @Value as BigInt ) returns VarChar(15) as
  begin
  -- Notes: 'A' = 0.  Negative numbers are not handled.
  declare @Result as VarChar(15) = '';

  if @Value = 0
    select @Result = 'A';
  else
    set @Value += 1;
  while @Value > 0
    select @Value -= 1, @Result = Char( ASCII( 'A' ) + @Value % 26 ) + @Result, @Value /= 26;
  return @Result;
  end;

Sample values:

select Arabic, dbo.HinkyBase26( Arabic ) as Alpha
  from ( values ( 0 ), ( 1 ), ( 25 ), ( 26 ), ( 51 ), ( 52 ),
    ( 27 * 26 - 1 ), ( 27 * 26 ),
    ( 33685567531 ) ) as Foo( Arabic );

At ErikE's suggestion I ran a quick performance test on my notebook. 1,000,000 iterations of the UDF vs. the XML solution:

declare @Count as Int;
declare @Buffer as VarChar(16);
declare @Start as DateTime;

select @Count = 1000000, @Start = GetDate();
while @Count > 0
  select @Buffer = dbo.HinkyBase26( @Count ), @Count -= 1;
select DateDiff( ms, @Start, GetDate() ) as 'Elapsed Milliseconds'; -- 14,583    
select @Count = 1000000, @Start = GetDate();
while @Count > 0
  select @Buffer =
      (
         SELECT Char( ASCII( 'A' ) + (@Count - X.Low) / X.Div % 26)
         FROM
            (
               SELECT 18279, 475254, 17576
               UNION ALL SELECT 703, 18278, 676
               UNION ALL SELECT 27, 702, 26
               UNION ALL SELECT 1, 26, 1
            ) X (Low, High, Div)      
         WHERE @Count >= X.Low
         FOR XML PATH(''), TYPE
      ).value('.[1]', 'varchar(4)'), @Count -= 1;
select DateDiff( ms, @Start, GetDate() ) as 'Elapsed Milliseconds'; -- 47,256

The UDF was a little more than 3 times faster.