在mysql中，通过一张表的列修改另一张关联表中的内容：
1: 修改1列
update student s, city c
set s.city_name = c.name
where s.city_code = c.code;
2: 修改多个列
update a, b
set a.title=b.title, a.name=b.name
where a.id=b.id
3: 采用子查询
update student s set city_name = (select name from city where code = s.city_code);
REF:
http://*.com/questions/6393763/fast-cross-table-update-with-mysql
http://*.com/questions/11709043/mysql-update-column-with-value-from-another-table