Parseo en MySQL -> Json, con n Registros

CREATE TABLE IF NOT EXISTS `user_groups` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
  `code` varchar(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
  `description` text CHARACTER SET utf8 COLLATE utf8_unicode_ci,
  `created_at` timestamp NULL DEFAULT NULL,
  `updated_at` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `user_groups_code_index` (`code`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8 COLLATE=utf8_spanish_ci;

DELETE FROM `user_groups`;
INSERT INTO `user_groups` (`id`, `name`, `code`, `description`, `created_at`, `updated_at`) VALUES
    (1, 'Guest', 'guest', 'Default group for guest users.', '2017-11-14 03:33:04', '2017-11-14 03:33:04'),
    (2, 'Registered', 'registered', 'Default group for registered users.', '2017-11-14 03:33:04', '2017-11-14 03:33:04'),
    (3, 'Auditores', 'auditores', 'Son los usuarios habilitados a Autorizar las publicaciones de los usuarios.', '2017-11-17 22:18:02', '2017-11-17 22:18:15'),
    (4, 'Estandar', 'estandar', 'Es el grupo de los usuarios estándares.', '2017-11-17 23:04:05', '2017-11-17 23:04:05');

CREATE DEFINER=`root`@`localhost` PROCEDURE `demoJson`()
    LANGUAGE SQL
    NOT DETERMINISTIC
    CONTAINS SQL
    SQL SECURITY DEFINER
    COMMENT 'AS'
BEGIN
  DECLARE myVariableJson VARCHAR(100);
  DECLARE done INT DEFAULT FALSE;
  DECLARE oneRecord VARCHAR(200);
  DECLARE allRecord VARCHAR(600);
  DECLARE nIndex    INT(3);
  DECLARE cur1 CURSOR FOR SELECT JSON_OBJECT("pkey",id, "nombre",name, "codigo",code) AS campito1
                                            FROM user_groups;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN cur1;

    SET nIndex:=0;
    read_loop: LOOP
    FETCH cur1 INTO oneRecord;
        IF done THEN
            LEAVE read_loop;
        END IF;

        SET nIndex:=nIndex+1;
        IF nIndex=1 THEN
            SET allRecord := oneRecord;
        ELSE
            SELECT JSON_MERGE(allRecord, oneRecord) INTO allRecord;
        END IF;

    END LOOP;

    SET myVariableJson := JSON_OBJECT('variable1', 'value1','variable2','value2');

    SELECT JSON_MERGE(myVariableJson, allRecord) ;
    CLOSE cur1;

END
CALL `demoJson`();

---- resultado ----
{"pkey": [1, 2, 3, 4], "codigo": ["guest", "registered", "auditores", "estandar"], "nombre": ["Guest", "Registered", "Auditores", "Estandar"], "variable1": "value1", "variable2": "value2"}