插入的数据如下：
CREATE TABLE IF NOT EXISTS `tab1` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`names` varchar(250) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ;
INSERT INTO `tab1` (`id`, `names`) VALUES
(1, '景临境'),
(2, '吴者然'),
(3, '暴天明'),
(4, '落月清');
CREATE TABLE IF NOT EXISTS `tab2` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`username` varchar(250) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ;
INSERT INTO `tab2` (`id`, `username`) VALUES
(1, 'xxx'),
(2, 'xxx'),
(3, 'xxx'),
(4, 'xxx');
执行该语句 UPDATE SET WHERE ：
UPDATE tab1,tab2 SET tab2.username = tab1.names WHERE tab1.id = tab2.id;
效果图：