将所有to_date为9999-01-01的全更新为NULL
将所有to_date为9999-01-01的全部更新为NULL
https://www.nowcoder.com/practice/859f28f43496404886a77600ea68ef59
update titles_test set to_date=null,from_date='2001-01-01' /* 更新为空值,columnname=null,不是columnname is null update tablename set columnname1=值,columnname2=值,... where condition */