Solution 1: 1列

update student s, city c
set s.city_name = c.name
where s.city_code = c.code;

Solution 2: 多个列

update a, b 
set a.title=b.title, a.name=b.name
where a.id=b.id

Solution 3: 子查询

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

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