If the corresponding primary key has been modified, use this option to set the &quot; IS NULL &quot; value to all external key fields. IS NULL means that the field is empty.
如果修改了相应主关键字，则使用此选项可将所有对应的外部关键字段设置为 &quot; IS NULL &quot; 。此处 &quot; IS NULL &quot; 表示该字段为空。