declare @str varchar(8000);
set @str = 'qothobnslfdnoweavnivmnqpwf';

;WITH
TENS      (N) AS (SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL 
                  SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0 UNION ALL SELECT 0),
THOUSANDS (N) AS (SELECT 1 FROM TENS t1 CROSS JOIN TENS t2 CROSS JOIN TENS t3),
MILLIONS  (N) AS (SELECT 1 FROM THOUSANDS t1 CROSS JOIN THOUSANDS t2),
TALLY     (N) AS (SELECT ROW_NUMBER() OVER (ORDER BY (SELECT 0)) FROM MILLIONS)
SELECT N, 
       LetterAsPosition = substring(@str, N, 1),
       AsciiAtPosition  = ascii(substring(@str, N, 1))
  FROM TALLY 
 WHERE N <= datalength(@str)
 ORDER BY N;

--=============================================================================
--      Create some test data. This is NOT a part of the solution
--=============================================================================
--===== Conditionally drop the test table to make reruns easier
     IF OBJECT_ID('tempdb..#MyHead','U') IS NOT NULL
        DROP TABLE #MyHead
;
--===== Build and populate the test table on the fly
WITH 
cteBuild3CharData AS
( --=== Builds 3 random characters and a random 1-3 length
 SELECT TOP 100000
        WierdData = CAST(CHAR(ABS(CHECKSUM(NEWID()))%256) AS VARCHAR(3))
                  + CAST(CHAR(ABS(CHECKSUM(NEWID()))%256) AS VARCHAR(3))
                  + CAST(CHAR(ABS(CHECKSUM(NEWID()))%256) AS VARCHAR(3)),
        RandomLen = ABS(CHECKSUM(NEWID()))%3+1
   FROM sys.all_columns ac1,
        sys.all_columns ac2
) --=== Use the random length to determine how long a string to save
     -- and put it all in the test table
 SELECT WierdData = SUBSTRING(WierdData,1,RandomLen)
   INTO #MyHead
   FROM cteBuild3CharData
;
--=============================================================================
--      In the absence of clear requirements, solve most of the problem.
--=============================================================================
--===== Display the Original data and 0-255 for each available character
 SELECT WierdData,
        Character1 = ASCII(SUBSTRING(WierdData,1,1)),
        Character2 = ASCII(SUBSTRING(WierdData,2,1)),
        Character3 = ASCII(SUBSTRING(WierdData,3,1))
   FROM #MyHead
;