update restaurant

    set AverageCost=null

    where AverageCost=-1

    _____________________________________________________________________________________

    update restarant

    set AverageCost=null

    where dbID in (select dbID

                   from restaurant

                   where AverageCost=-1)

    以上都是错的。。。。

    1. 执行时会错误,报:

           You are using safe update mode and you tried to update a table without a WHERE clause that  

           uses a KEY column.”      

       原因是在safe mode下,要强制安全点,update只能跟where了,要取消这个限制,可以:
           SET SQL_SAFE_UPDATES=0;

    2. 执行时会错误,报:

           You can't specify target table 'restaurant' for update in FROM clause

       mysql中不能先select出同一表中的某些值，再update这个表(在同一语句中) 

    正确update方法：

    1. create table tmp as select dbID as ID from restaurant where AverageCost=-1;

       update restaurant set AverageCost=null where dbID in (select ID from tmp);

       drop table tmp;

       先生成一个临时表，在update，但是又遇到了问题，它太慢太慢太慢了，可能是因为有表连接。。。换第二种方法

    2.  update restaurant

       inner join tmp on dbID=ID

       set AverageCost=null