DELIMITER //  
DROP FUNCTION IF EXISTS `rand_string` //  
CREATE FUNCTION `rand_string`(n INT) RETURNS varchar(255) CHARSET utf8  
BEGIN  
    DECLARE chars_str varchar(255) DEFAULT 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';  
    DECLARE return_str varchar(255) DEFAULT '';  
    DECLARE i INT DEFAULT 0;  
    WHILE i < n DO  
        SET return_str = concat(return_str,substring(chars_str , FLOOR(1 + RAND()*62 ),1));  
        SET i = i + 1;  
    END WHILE;  
    RETURN return_str;  
END//  

UPDATE yp_express_users SET user_token = rand_string(32) WHERE 1;