update websites set master = 2 where url = select url from websites where id = 12;
显然，mysql不允许您对正在更新的表运行选择查询。
将其放入派生表中这被具体化到临时表中，并绕过了限制。
update websites
set master = 2
where url in (select url
from (select url
from websites
where id = 12) t);
update websites set master = 2 where url in (select w2.url from websites w2 where w2.id = 12);
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