用一個表中的欄位去更新另外一個表中的欄位，來看一個簡單的例子就會了：

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');
有兩個表：student & city，現在需要取出 city.name 來更新 student.city_name。兩表關聯條件是 student.city_code=city.code。

update
 
 student s, city c
   set s.city_name = c.name
 where s.city_code = c.code;
也可以試下面的相關子查詢：

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