What describes Delete Set-to-Null?

What describes Delete Set-to-Null?

• A. When the parent is deleted, the foreign key values in referencing records are set to NULL.
• B. Deleting a parent always deletes all children.
• C. It prevents deletion of the parent when there are references.
• D. It changes the data type of the foreign key.

Answer: (A)

Delete Set-to-Null means that when a row in the parent table is deleted, the corresponding foreign key values in the child table are set to NULL. This preserves the child rows but removes the link to the now-deleted parent. For this to work, the foreign key column must be nullable, because you can’t set a value to NULL if the column is defined as NOT NULL. For example, if an orders table has a customer_id that references a customers table, deleting a customer will set the customer_id in those orders to NULL, rather than deleting the orders or blocking the delete. This behavior is different from deleting the parent and cascading the delete to children (which would remove the child rows), or from preventing deletion when there are references (which stops the delete), while changing the data type has nothing to do with how deletions propagate.

Delete Set-to-Null means that when a row in the parent table is deleted, the corresponding foreign key values in the child table are set to NULL. This preserves the child rows but removes the link to the now-deleted parent. For this to work, the foreign key column must be nullable, because you can’t set a value to NULL if the column is defined as NOT NULL.

For example, if an orders table has a customer_id that references a customers table, deleting a customer will set the customer_id in those orders to NULL, rather than deleting the orders or blocking the delete.

This behavior is different from deleting the parent and cascading the delete to children (which would remove the child rows), or from preventing deletion when there are references (which stops the delete), while changing the data type has nothing to do with how deletions propagate.