假设a,b两表有外键关联，想要从b表中取出相应字段的值更新a表字段，可以有如下几种写法：

1、update a set a.name=b.name from a,b where a.id=b.id

2、update a inner join b on a.id=b.id set a.name=b.name where ...

3、update table1 set a.name = b.name from table1 a inner join table2 b on a.id =b.id where...