Foreign key ON UPDATE behavior in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a foreign key in a database?

A foreign key is a column or set of columns in one table that refers to the primary key in another table. It creates a link between the two tables to maintain data consistency.

beginner

What does the ON UPDATE clause do in a foreign key constraint?

The ON UPDATE clause defines what happens to the foreign key values when the referenced primary key is updated. It controls how changes in the parent table affect the child table.

intermediate

Explain the ON UPDATE CASCADE behavior.

ON UPDATE CASCADE means that when the primary key in the parent table is updated, the foreign key values in the child table are automatically updated to match the new value.

intermediate

What happens with ON UPDATE SET NULL?

With ON UPDATE SET NULL, if the primary key in the parent table is updated, the foreign key values in the child table are set to NULL, indicating no reference.

advanced

Describe the difference between ON UPDATE RESTRICT and ON UPDATE NO ACTION.

ON UPDATE RESTRICT prevents the update of the primary key if there are matching foreign keys in the child table. ON UPDATE NO ACTION also prevents the update but checks the constraint at the end of the statement, which can allow deferred checks.

What does ON UPDATE CASCADE do in a foreign key constraint?

A Sets foreign keys to NULL when parent key changes

B Deletes rows in child table when parent key changes

C Updates foreign keys in child table when parent key changes

D Prevents updates to parent key if child rows exist

ON UPDATE CASCADE automatically updates the foreign key values in the child table to match the new primary key value in the parent table.

If a foreign key has ON UPDATE SET NULL, what happens when the parent key is updated?

A Foreign key values are deleted

B Foreign key values are set to NULL

C Update is blocked

D Foreign key values are unchanged

ON UPDATE SET NULL sets the foreign key values to NULL when the referenced primary key is updated.

Which ON UPDATE option prevents the parent key from being updated if child rows exist?

A CASCADE

B SET NULL

C SET DEFAULT

D RESTRICT

ON UPDATE RESTRICT blocks the update of the parent key if there are matching foreign keys in the child table.

What is the difference between ON UPDATE NO ACTION and ON UPDATE RESTRICT?

A NO ACTION checks constraints at statement end, RESTRICT checks immediately

B NO ACTION allows updates, RESTRICT blocks them

C NO ACTION deletes child rows, RESTRICT sets them NULL

D They behave exactly the same

ON UPDATE NO ACTION defers constraint checking until the end of the statement, while RESTRICT checks immediately and blocks the update if violated.

Which ON UPDATE behavior automatically updates child rows to match changes in the parent key?

A CASCADE

B SET NULL

C RESTRICT

D NO ACTION

CASCADE updates child rows automatically when the parent key changes.

Explain the different ON UPDATE behaviors available for foreign keys and when you might use each.

Think about how changes in the parent table should affect related child rows.

Describe a real-life example where ON UPDATE CASCADE would be helpful.

Imagine changing an ID that is used in many places.