Which option describes Update Set-to-Default?

Which option describes Update Set-to-Default?

• A. Updates to the parent key are blocked if there are references.
• B. Referencing keys are updated to the new value.
• C. On updating the primary key, referencing keys are set to NULL.
• D. On updating the primary key, referencing keys are set to the default value.

Answer: (D)

Update Set-to-Default happens when a referenced primary key changes and the foreign key constraint uses ON UPDATE SET DEFAULT. In this scenario, every child row that points to the old parent key will have its foreign key value replaced by the default value defined for that column. This preserves referential integrity by removing the old reference and replacing it with a predefined default, rather than propagating the new key or clearing the reference. For this to work, a default value must exist for the foreign key column, and not all databases support this action. This behavior is different from updating to the new value (where children mirror the parent’s new key) and from setting to NULL (where children become NULL).

Update Set-to-Default happens when a referenced primary key changes and the foreign key constraint uses ON UPDATE SET DEFAULT. In this scenario, every child row that points to the old parent key will have its foreign key value replaced by the default value defined for that column. This preserves referential integrity by removing the old reference and replacing it with a predefined default, rather than propagating the new key or clearing the reference. For this to work, a default value must exist for the foreign key column, and not all databases support this action. This behavior is different from updating to the new value (where children mirror the parent’s new key) and from setting to NULL (where children become NULL).