CREATE FUNCTION dbo.NextTicket( @Ticket VARCHAR(128) ) RETURNS VARCHAR(128) AS
BEGIN

   DECLARE @digits TABLE( value INT IDENTITY(0,1) NOT NULL PRIMARY KEY, symbol CHAR(1) NOT NULL UNIQUE );
   DECLARE @place INT, @zero_digit CHAR(1), @highest_digit CHAR(1);
   
-- 1. build a table of symbols that compose the "digits" of our ticket numbers
--    Note that you could impose artifical order, it just happens that sorting by
--    the value gives me the order I want.
   INSERT INTO @digits ( symbol )
   SELECT ch
   FROM (
       SELECT '0' AS ch UNION SELECT '1' UNION SELECT '2' UNION SELECT '3' UNION SELECT '4' UNION SELECT '5' 
       UNION SELECT '6' UNION SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT 'A' UNION SELECT 'B' 
       UNION SELECT 'C' UNION SELECT 'D' UNION SELECT 'E' UNION SELECT 'F' UNION SELECT 'G' UNION SELECT 'H' 
       UNION SELECT 'I' UNION SELECT 'J' UNION SELECT 'K' UNION SELECT 'L' UNION SELECT 'M' UNION SELECT 'N' 
       UNION SELECT 'O' UNION SELECT 'P' UNION SELECT 'Q' UNION SELECT 'R' UNION SELECT 'S' UNION SELECT 'T' 
       UNION SELECT 'U' UNION SELECT 'V' UNION SELECT 'W' UNION SELECT 'X' UNION SELECT 'Y' UNION SELECT 'Z'
   ) AS symbols
   ORDER BY ch ASC;

   SELECT       @zero_digit    = symbol FROM @digits WHERE value = 0;
   SELECT TOP 1 @highest_digit = symbol FROM @digits ORDER BY value DESC;


-- 2. assume valid input (functions can't throw errors) -----------------------
   SET @Ticket = RTRIM(LTRIM(@Ticket));


-- 3. carry the 1 -------------------------------------------------------------
   SET @place = LEN(@Ticket);

   WHILE @place > 0 AND SUBSTRING(@Ticket, @place, 1) = @highest_digit
   BEGIN

       SET @Ticket = STUFF(@Ticket, @place, 1, @zero_digit);
       SET @place = @place -1;

   END


-- 4. final increment ---------------------------------------------------------
   IF @place = 0 BEGIN
       DECLARE @one_digit CHAR(1), @length INT;        
       SELECT @one_digit = symbol FROM @digits WHERE value = 1;
   
       SET @length = LEN(@Ticket);
       SET @Ticket = @one_digit + @Ticket; -- if this overflows, it will truncate the right most "digit"

       IF @length = LEN(@Ticket) -- still the same length only if overflowed...
           SET @Ticket = REPLICATE(@zero_digit, @length); -- roll over back to zero

   END ELSE BEGIN
       DECLARE @next_digit CHAR(1);
       SET @next_digit = (
           SELECT symbol FROM @digits WHERE value = (
               SELECT value +1 FROM @digits WHERE symbol = SUBSTRING(@Ticket, @place, 1)));
            
       SET @Ticket = STUFF(@Ticket, @place, 1, @next_digit);
   END

-- 5. Return the new Ticket number
   RETURN @Ticket;
END

 

DBCC DROPCLEANBUFFERS 
GO
SET STATISTICS IO ON
SET STATISTICS TIME ON

-----------------------------------------------------------------------------------------
RESULTS                       Elapsed    |   Statements Run | Logical Read   | Scans | CPU              
-----------------------------------------------------------------------------------------
Using table                      126 ms   |       23               |    118            |    4    |   48
Using string                        0 ms    |       11               |      0             |    0    |    0

/*  Remove any prior version of function  */

IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[dbo].[fn_NextSerial]') 
	AND xtype IN (N'FN', N'IF', N'TF'))
DROP FUNCTION [dbo].[fn_NextSerial]
GO

CREATE FUNCTION fn_NextSerial (@currentboth CHAR(2)) 
RETURNS char(2)
AS

/*  Given the current value of a two-character string (@currentboth),
 	return the next higher value based on the string of valid chars (@alphas) 

	Example:  fn_NextSerial('AZ') will return 'BA'

	Note:  If the string can not be incremented any further, '**' is returned
               If the string contains invalid characters, '--' is returned
*/

BEGIN   

   DECLARE @alphas CHAR(36) -- valid alpha values (length must match that of string assigned below)
   SET @alphas = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' --  valid chars for incrementing string

   DECLARE @current1 INT  -- index in valid alpha values of 1st char in string
   DECLARE @current2 INT  -- index in valid alpha values of 2nd char in string

   SET @current1 = CHARINDEX(SUBSTRING(@currentboth,1,1),@alphas)
   SET @current2 = CHARINDEX(SUBSTRING(@currentboth,2,1),@alphas)

   /*  If any invalid or null input, return '--'  */
   IF @current1 = 0 OR @current2 = 0 OR @currentboth IS NULL
	SET @currentboth = '--'
   ELSE

   BEGIN
     IF @current2 = LEN(@alphas) --  Is the 2nd char at the last valid string position?  
     BEGIN
	IF @current1 <> LEN(@alphas) --  Can the 1st char still be incremented? 
	BEGIN
		/* Increment the 1st char position and set the 2nd char to starting char */
		SET @currentboth  =  SUBSTRING(@alphas,(@current1+1),1)+
		SUBSTRING(@alphas,1,1)
	END
	ELSE
		SET @currentboth = '**' --  return '**' to show no more incrementing possible
     END
     ELSE
	/*  Leave 1st char unchanged and increment the 2nd char of the string */
	SET  @currentboth= SUBSTRING(@currentboth,1,1)  + SUBSTRING(@alphas,@current2+1,1)
  END

  RETURN @currentboth -- return the next value or 
                      -- '**' if already at highest value or 
	              -- '--' if invalid chars passed
  END
GO

Select 'Next after 00 is:' + dbo.fn_NextSerial('00') -- '01'
Select 'Next after 0Y is:' + dbo.fn_NextSerial('0Y') -- '0Z'
Select 'Next after 0Z is:' + dbo.fn_NextSerial('0Z') -- '10'
Select 'Next after ZY is:' + dbo.fn_NextSerial('ZY') -- 'ZZ'
Select 'Next after ZZ is:' + dbo.fn_NextSerial('ZZ') -- '**'
Select 'Next after '' Z'' is:' + dbo.fn_NextSerial(' Z') -- '--'
Select 'Next after @0 is:' + dbo.fn_NextSerial('@0') -- '--'
Select 'Next after A$ is:' + dbo.fn_NextSerial('A$') -- '--'
Select 'Next after null is:' + dbo.fn_NextSerial(NULL) -- '--'

--DBCC DROPCLEANBUFFERS  -- clear the buffers 
--go

/*   Prep as if we are calling the function */

   declare @Ticket varchar(128)
   set statistics io on
   set statistics time on
   set @Ticket = 'ZZZ'  -- change this and rerun to test different cases
   select 'Input is ' + @Ticket

/*  Invoke the statements from the body of the function  */

  -- DECLARE @digits TABLE( value INT IDENTITY(0,1) NOT NULL PRIMARY KEY, symbol CHAR(1) NOT NULL UNIQUE );
   DECLARE @place INT, @zero_digit CHAR(1), @highest_digit CHAR(1);
   
-- 1. build a table of symbols that compose the "digits" of our ticket numbers
--    Note that you could impose artifical order, it just happens that sorting by
--    the value gives me the order I want.
 /*  INSERT INTO @digits ( symbol )
   SELECT ch
   FROM (
       SELECT '0' AS ch UNION SELECT '1' UNION SELECT '2' UNION SELECT '3' UNION SELECT '4' UNION SELECT '5' 
       UNION SELECT '6' UNION SELECT '7' UNION SELECT '8' UNION SELECT '9' UNION SELECT 'A' UNION SELECT 'B' 
       UNION SELECT 'C' UNION SELECT 'D' UNION SELECT 'E' UNION SELECT 'F' UNION SELECT 'G' UNION SELECT 'H' 
       UNION SELECT 'I' UNION SELECT 'J' UNION SELECT 'K' UNION SELECT 'L' UNION SELECT 'M' UNION SELECT 'N' 
       UNION SELECT 'O' UNION SELECT 'P' UNION SELECT 'Q' UNION SELECT 'R' UNION SELECT 'S' UNION SELECT 'T' 
       UNION SELECT 'U' UNION SELECT 'V' UNION SELECT 'W' UNION SELECT 'X' UNION SELECT 'Y' UNION SELECT 'Z'
   ) AS symbols
   ORDER BY ch ASC;
*/
	DECLARE @digits CHAR(36) -- valid alpha values (length must match that of string assigned below)
SET @digits = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ' --  valid chars for incrementing string
	

  -- SELECT       @zero_digit    = symbol FROM @digits WHERE value = 0;
   SET @zero_digit = SUBSTRING(@digits,1,1)
  --   SELECT TOP 1 @highest_digit = symbol FROM @digits ORDER BY value DESC;
   SET @highest_digit = SUBSTRING(@digits,LEN(@digits),1)

-- 2. assume valid input (functions can't throw errors) -----------------------
      SET @Ticket = RTRIM(LTRIM(@Ticket));
  


-- 3. carry the 1 -------------------------------------------------------------
   SET @place = LEN(@Ticket);

   WHILE @place > 0 AND SUBSTRING(@Ticket, @place, 1) = @highest_digit
   BEGIN

       SET @Ticket = STUFF(@Ticket, @place, 1, @zero_digit);
       SET @place = @place -1;

   END


-- 4. final increment ---------------------------------------------------------
   IF @place = 0 BEGIN
       DECLARE @one_digit CHAR(1), @length INT;        
     --  SELECT @one_digit = symbol FROM @digits WHERE value = 1;
	SET @one_digit = SUBSTRING(@digits,2,1)
   
       SET @length = LEN(@Ticket);
       SET @Ticket = @one_digit + @Ticket; -- if this overflows, it will truncate the right most "digit"

       IF @length = LEN(@Ticket) -- still the same length only if overflowed...
           SET @Ticket = REPLICATE(@zero_digit, @length); -- roll over back to zero

   END 
   ELSE BEGIN
       DECLARE @next_digit CHAR(1);
        --SET @next_digit = 
        --   (SELECT symbol FROM @digits WHERE value = (
        --      SELECT value +1 FROM @digits WHERE symbol = SUBSTRING(@Ticket, @place, 1)));
	
       SET @next_digit =   
            SUBSTRING(@digits,CHARINDEX(SUBSTRING(@Ticket,@place,1),@digits)+1,1)           

       SET @Ticket = STUFF(@Ticket, @place, 1, @next_digit);

   END

/*  Put out the result */
select 'Answer is ' + @Ticket
set statistics io off
set statistics time off

ALTER FUNCTION dbo.Base36Add( @value1 varchar(128), @value2 varchar(128) ) RETURNS varchar(128) AS
BEGIN

	DECLARE @digits char(36); SET @digits = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ';
	DECLARE @sum varchar(16), @i int, @s int, @d1 int, @d2 int, @r int;

-- 1. null values are invalid -------------------------------------------------
	IF @value1 IS NULL OR @value2 IS NULL
		GOTO Exit_Error;

-- 2. pad so they are the same length -----------------------------------------	
	IF len(@value1) < len(@value2)
		SET @value1 = replicate( substring(@digits,1,1), len(@value2)-len(@value1) ) + @value1;
	ELSE IF len(@value2) < len(@value1)
		SET @value2 = replicate( substring(@digits,1,1), len(@value1)-len(@value2) ) + @value2;


-- 3. sum and carry -----------------------------------------------------------
	SELECT 
		@sum = '',         /* init string for prepending */
		@i = len(@value1), /* start at least signifigant place */
		@r = 0;            /* start with no remainder */

	WHILE 0 < @i BEGIN /* for each place */
	-- a. convert the sum of the remainder plus the current place in both values to an integer
		SELECT 
			@d1 = charindex( substring(@value1,@i,1), @digits ) - 1,
			@d2 = charindex( substring(@value2,@i,1), @digits ) - 1;

		IF (coalesce(@d1,-1) = -1 OR coalesce(@d2,-1) = -1)
			GOTO Exit_Error;

		SET @s = @r + @d1 + @d2;
	
	-- b. find the resulting total for this place
		SET @sum = substring(@digits, (@s % 36) + 1, 1) 
                 + @sum;
	
	-- c. find the remainder to carry forward
		SET @r = (@s / 36);
	
	-- d. next place...
		SET @i = @i - 1;

	END


-- 4. final remainder?
	IF 0 < @r BEGIN
		IF len(@sum) < 128 /* defined return value size */
			SET @sum = substring(@digits, @r + 1, 1) + @sum;
		ELSE
			SET @sum = '**'; /* overflow */
	END


Exit_Success:
	RETURN @sum;

Exit_Error:
	RETURN '--'; /* invalid input */

END

CREATE FUNCTION [dbo].[NextTicket]( @Ticket varchar(2) ) RETURNS varchar(2) AS
BEGIN

	IF @Ticket = 'ZZ'
		RETURN '**';

	RETURN dbo.Base36Add( @Ticket, '1' );

END

SELECT
	dbo.NextTicket('00') AS Expect_01,
	dbo.NextTicket('0Y') AS Expect_0Z,
	dbo.NextTicket('0Z') AS Expect_10,
	dbo.NextTicket('ZY') AS Expect_ZZ,
	dbo.NextTicket('ZZ') AS Expect_Overflow,
	dbo.NextTicket(' Z') AS Expect_Invalid_01,
	dbo.NextTicket('@0') AS Expect_Invalid_02,
	dbo.NextTicket('A$') AS Expect_Invalid_03,
	dbo.NextTicket(NULL) AS Expect_Invalid_04

SELECT
	dbo.fn_NextSerial('00') AS Expect_01,
	dbo.fn_NextSerial('0Y') AS Expect_0Z,
	dbo.fn_NextSerial('0Z') AS Expect_10,
	dbo.fn_NextSerial('ZY') AS Expect_ZZ,
	dbo.fn_NextSerial('ZZ') AS Expect_Overflow,
	dbo.fn_NextSerial(' Z') AS Expect_Invalid_01,
	dbo.fn_NextSerial('@0') AS Expect_Invalid_02,
	dbo.fn_NextSerial('A$') AS Expect_Invalid_03,
	dbo.fn_NextSerial(NULL) AS Expect_Invalid_04