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当ON DELETE SET NULL规则被激活时，如果一条记录从父表中删除，子表中的所有记录以及匹配的外键值都被设置为NULL。
When ON delete set NULL is active, when a record is deleted from the parent table, all records in the child table with matching foreign key values are set to NULL.
更新一条特殊记录包括获取这个记录的句柄以及设置新信息。
Updating a particular record involves getting a handle for that record and setting new information.
在设置一个值后，键就从数据库中移除掉，并添加一条具有更新过的值的新记录。
Then when a value is set, the key is removed from the database and a new record is added with the updated value.
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