You could create a mapping table with two columns "index","char" then you could select based on rand() from the mapping table, without the need to do any "if" or "calculation"

Maybe you want RAND() to be deterministic, but you will be lonely :)

The RAND function is treated as a non-deterministic function. The query optimizer does not cache the results of the RAND function.

http://dcx.sybase.com/index.html#1200en/dbreference/rand-function.html

Once upon a time (I think) RAND was deterministic, but that bug got fixed.

FWIW here's the code I use...

--------------------------------------------------------------------- 
BEGIN
   DROP FUNCTION generate_random_string;
   EXCEPTION WHEN OTHERS THEN
END;

CREATE FUNCTION generate_random_string (
   IN @return_length INTEGER,
   IN @character_set VARCHAR ( 100 ) DEFAULT '' )
   RETURNS LONG BINARY
   NOT DETERMINISTIC
BEGIN
   DECLARE @characters_to_use   LONG BINARY;
   DECLARE @character_count     DOUBLE;
   DECLARE @random_number       DOUBLE;
   DECLARE @return_position     INTEGER;
   DECLARE @character_position  INTEGER;
   DECLARE @return_value        LONG BINARY;

CASE @character_set

WHEN 'caps' THEN
         SET @characters_to_use = STRING ( 
            'QWERTYUIOPASDFGHJKLZXCVBNM' );

WHEN 'letters' THEN
         SET @characters_to_use = STRING ( 
            'QWERTYUIOPASDFGHJKLZXCVBNM',
            'qwertyuiopasdfghjklzxcvbnm' );

WHEN 'numbers' THEN
         SET @characters_to_use = STRING ( 
            '1234567890' );

WHEN 'alphanumeric' THEN
         SET @characters_to_use = STRING ( 
            'QWERTYUIOPASDFGHJKLZXCVBNM',
            'qwertyuiopasdfghjklzxcvbnm',
            '1234567890' );

WHEN 'loweralphanumeric' THEN
         SET @characters_to_use = STRING ( 
            'qwertyuiopasdfghjklzxcvbnm',
            '1234567890' );

WHEN 'upperalphanumeric' THEN
         SET @characters_to_use = STRING ( 
            'QWERTYUIOPASDFGHJKLZXCVBNM',
            '1234567890' );

ELSE
         SET @characters_to_use = STRING ( -- omitted: single quote, back slash
            'QWERTYUIOPASDFGHJKLZXCVBNM',
            'qwertyuiopasdfghjklzxcvbnm',
            '1234567890',
            '`~-=!@#$%^&*()_+{}|[]:"<>?;,./' );

END CASE;

SET @character_count = LENGTH ( @characters_to_use );

SET @random_number = RAND();

SET @return_position = 1;
   WHILE @return_position <= @return_length LOOP
      SET @character_position = CAST ( ROUND ( @random_number * @character_count, 0 ) AS INTEGER );
      SET @return_value = STRING ( @return_value, SUBSTR ( @characters_to_use, @character_position, 1 ) );
      -- MESSAGE STRING ( @return_position, ', ', @random_number, ', ', @character_position, ', ', @return_value ) TO CLIENT;
      SET @random_number = RAND();
      SET @return_position = @return_position + 1;
   END LOOP;

RETURN @return_value;

END; -- generate_random_string

--------------------------------------------------------------------- 
-- Testing...

/*

BEGIN
   DECLARE @seed        INTEGER;
   DECLARE @dummy       DOUBLE;

-- Note: DATEDIFF returns INTEGER so the range of @seed is limited.
   --       However, this logic will work well past the year 2055 since
   --       a negative seed is OK.

-- Note: The extra "seeding" crap may no longer be necessary in V12.

SET @seed = DATEDIFF ( SECOND, CURRENT TIMESTAMP, '2055 10 27 00:00:00' );
   SET @dummy = RAND ( @seed );
   SET @dummy = RAND();
   SET @dummy = RAND();

SELECT CAST ( generate_random_string ( 5 ) AS VARCHAR )                       AS "1",
          CAST ( generate_random_string ( 5, 'caps' ) AS VARCHAR )               AS "2",
          CAST ( generate_random_string ( 16, 'alphanumeric' ) AS VARCHAR )      AS "3",
          CAST ( generate_random_string ( 15, 'loweralphanumeric' ) AS VARCHAR ) AS "4",
          CAST ( generate_random_string ( 30, 'upperalphanumeric' ) AS VARCHAR ) AS "5";

END;

1,2,3,4,5
'yk2{j','XGBCL','SKJhKeOmxhAhGJWF','dxw4oef9rkocygh','FDQGCWZ6RIZQMJE447Y7BP84N0QOK8'

*/