For relationships enforcing referential integrity between tables, cascading NULL is an option that causes a deleted row in the primary table to automatically update the foreign key in all related records in the related foreign table or tables with a NULL value.
Now also definable from SQL DDL.
For example, if you establish a relationship between a Customers (primary) table and an Orders (foreign) table with the cascading NULL option enabled, when the primary key in a record in the Customers table is deleted, all orders in the foreign Orders table associated with that customer would also be changed to have a NULL value.