UPDATE DB1.A SET a = ( SELECT b FROM DB2.B WHERE B.Id = A.id)
实例：
UPDATE wordpress.`wp_posts` SET post_content = ( SELECT `内容` FROM test.`content` WHERE content.Id = wp_posts.`ID`)
mysql：把DB1中A表a字段替换为DB2中B表b字段
原文：http://www.cnblogs.com/bushe/p/3956690.html
