Support UNION for ALTER TABLE

CREATE DATABASE IF NOT EXISTS `test_schema`;

USE `test_schema`;

CREATE TABLE `merge_table` (
  `id` int(11) NOT NULL,
  `name` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MRG_MyISAM DEFAULT CHARSET=latin1;

CREATE TABLE IF NOT EXISTS `table1` (
  `id` int(11) NOT NULL,
  `name` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;


CREATE TABLE  IF NOT EXISTS`table2` (
  `id` int(11) NOT NULL,
  `name` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

ALTER TABLE `merge_table` UNION = (`table1`,`table2`);