Foreign Key Delete Actions

NULL Child Key Does Not Reference

A NULL child foreign-key value points to no parent in this toy convention.

highlighted = computed this step

NULL child key does not reference

Under this toy convention, a NULL child foreign-key value points to no parent.

NULL no reference\text{NULL no reference}

Keep the NULL row

The reference table has 1 NULL child key, while cascade affects 2 matching child rows.

NULL child keys=1\text{NULL child keys}=1

Foreign-key delete-action examples are tiny finite table checks; SET NULL, deferrable checks, triggers, transactions, locks, timing, indexing, performance, general referential-action completeness, and product behavior are out of scope.

NULL child key does not reference: parent rowscustomer_idname1Ada2Ben3Cy4Dee NULL child key does not reference: child rowsorder_idcustomer_idtotal101150102230103170104NULL5 Child-to-parent referenceschildRowchildValueparentRowstatus010matches_parent121matches_parent210matches_parent3NULLnoneNULL_no_reference Delete resultfieldvalueactioncascadeparentValue1parentRow0statusdeletedchildRowsAffected(0, 2) Parent after deletecustomer_idname2Ben3Cy4Dee Child after deleteorder_idcustomer_idtotal102230104NULL5 FK delete factsfactvalueparentRowCount4childRowCount4referenceCount4nullChildKeyCount1affectedChildCount2deleteStatusdeletedparentAfterCount3childAfterCount2supportedActionsrestrict,cascade

Read NULL no reference

The NULL child row is not counted as referencing Ada.

status NULL no reference\text{status NULL no reference}

Summary

The NULL rule is labeled as this book's toy boundary.

explicit convention\text{explicit convention}