create table student

(

   student_id    int          not null

  ,student_name  varchar(30)  not null

  ,city_code     varchar(10)  null

  ,city_name     varchar(50)  null

);

create table city

(

   code varchar(10) not null

  ,name varchar(50) not null

);

insert into student values(1, 'john', '001', null);

insert into student values(2, 'nick', '002', null);

insert into city values('001', 'beijing');

insert into city values('002', 'shanghai');

insert into city values('003', 'shenzhen');

有两个表：student & city，现在需要取出 city.name 来更新 student.city_name。两表关联条件是 student.city_code=city.code。

update student s, city c

   set s.city_name = c.name

 where s.city_code = c.code;

也可以试下面的相关子查询：

update student s set city_name = (select name from city where code = s.city_code);