您要使用语句的insert ... select形式:

INSERT INTO data_stocks (data_id,quantity)
    select ud.data_id, $quantity
    from users u join
         users_data ud
         on u.uid = ud.uid;

如果您仅针对一个用户执行此操作，则它看起来可能更像这样:

If you are doing this for only one user, it might look more like this:

INSERT INTO data_stocks (data_id,quantity)
    select ud.data_id, $quantity
    from users_data ud
    where ud.uid = $uid;