How does mysql modify a column of data in table A to data in table b

Table Structure:

A表：
CREATE TABLE `bank_card_info_new` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `bank_code` varchar(19) DEFAULT NULL COMMENT '',
  `bank_name` varchar(255) DEFAULT NULL COMMENT '',
  `card_name` varchar(255) DEFAULT NULL COMMENT '',
  `card_length` varchar(2) DEFAULT NULL COMMENT '',
  `card_like` varchar(36) DEFAULT NULL COMMENT ',
  `card_num` varchar(19) DEFAULT NULL COMMENT '',
  `card_type` varchar(255) DEFAULT NULL COMMENT '',
  `bankserial` varchar(255) DEFAULT NULL COMMENT '',
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=5185 DEFAULT CHARSET=utf8 COMMENT='';

B表：
CREATE TABLE `bank_card_info` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `bank_code` varchar(19) DEFAULT NULL COMMENT '',
  `bank_name` varchar(255) DEFAULT NULL COMMENT '',
  `card_name` varchar(255) DEFAULT NULL COMMENT '',
  `card_length` varchar(2) DEFAULT NULL COMMENT '',
  `card_like` varchar(36) DEFAULT NULL COMMENT ',
  `card_num` varchar(19) DEFAULT NULL COMMENT '',
  `card_type` varchar(255) DEFAULT NULL COMMENT '',
  `bankserial` varchar(255) DEFAULT NULL COMMENT '',
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=5185 DEFAULT CHARSET=utf8 COMMENT='';

sql：

update
 bank_card_info_new t1,bank_card_info t2 
 set t1.bankserial = t2.bankserial 
 where t1.card_num = t2.card_num