Answer to Question 1
A checkpoint is a part of a recovery log. When a checkpoint is created, all updates recorded in the recovery logand as a result any updates to data stored in memoryare written to the database in the data file on the disk.
Answer to Question 2
The update set-to-null option allows a record to be changed if its primary key value is referred to by a foreign key value of a record in another relation. As a consequence, in all of the records where the foreign key value refers to the primary key being changed, the value of the foreign key is set to null.