MySQL UPDATE tb1 SET fn1=val WHERE fn IN (SELECT fn FROM tb2)

正确的写法：

--最先让人想到的写法
UPDATE buyer SET is_seller=1 WHERE uid IN (SELECT uid FROM seller) 

-- 网上的一种
UPDATE buyer a,seller b SET a.is_seller=1 WHERE a.uid=b.uid 

UPDATE tb1 t1 ,tb2 t2 set t1,fn1=t2.fn2 where t1.id=t2.id

重新整理：http://blog.youkuaiyun.com/default7/article/details/77477627