drop function if exists rand_string;  
create function rand_string(str_length tinyint unsigned, str_type tinyint unsigned) returns varchar(255)
begin
    -- Function   : rand_string
    -- Author     : reymondtu#opencfg.com
    -- Date       : 2011/03/27
    -- Params     : str_length int unsigned 
    --                  The random string length of random string
    --              str_type   int unsigned
    --                  The random string type
    --                      1.0-9
    --                      2.a-z
    --                      3.A-Z
    --                      4.a-zA-Z
    --                      5.0-9a-zA-Z
    --
    -- Example    :
    --
    -- mysql> select rand_string(32,5) from dual;
    -- +----------------------------------+
    -- | rand_string(32,5)                |
    -- +----------------------------------+
    -- | HbPBz4DWSAiJNLt4SgExHVwQI34bI6mt |
    -- +----------------------------------+
    -- 1 row in set

    declare counter int unsigned default 0;
    declare const_chars varchar(64) default '0123456789';
    declare result varchar(255) default '';
 
    if str_type = 1 then
        set const_chars = '0123456789';
    elseif str_type = 2 then
        set const_chars = 'abcdefghijklmnopqrstuvwxyz';
    elseif str_type = 3 then
        set const_chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    elseif str_type = 4 then
        set const_chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    elseif str_type = 5 then
        set const_chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    else
        set const_chars = '0123456789';
    end if;
 
    while counter < str_length do  
        set result = concat(result,substr(const_chars,ceil(rand()*(length(const_chars)-1)),1));
    set counter = counter + 1;
    end while;

    return result;
end