Foreign key ON UPDATE behavior in SQL - Step-by-Step Execution

Concept Flow - Foreign key ON UPDATE behavior

Start: Update parent key

↓

Check foreign key constraint

↓

ON UPDATE CASCADE

↓

Update child keys

↓

Commit changes

↓

End

When a parent key is updated, the database checks the foreign key rule and applies the ON UPDATE action to child rows accordingly.

Execution Sample

SQL

CREATE TABLE parent (
  id INT PRIMARY KEY
);

CREATE TABLE child (
  id INT PRIMARY KEY,
  parent_id INT,
  FOREIGN KEY (parent_id) REFERENCES parent(id) ON UPDATE CASCADE
);

UPDATE parent SET id = 2 WHERE id = 1;

This code creates two tables with a foreign key and updates the parent key to show ON UPDATE CASCADE effect.

Execution Table

Step	Action	Parent.id Before	Parent.id After	Child.parent_id Before	Child.parent_id After	Notes
1	Initial state	1	1	1	1	Parent and child have id=1
2	Update parent.id from 1 to 2	1	2	1	1	Parent id updated, child not yet updated
3	Check foreign key ON UPDATE CASCADE	2	2	1	2	Child.parent_id updated to 2 to match parent
4	Commit changes	2	2	2	2	Update complete, foreign key constraint maintained
5	End	2	2	2	2	Execution finished

💡 Update completes successfully with child keys updated due to ON UPDATE CASCADE

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
parent.id	1	2	2	2
child.parent_id	1	1	2	2

Key Moments - 2 Insights

Why does child.parent_id change after updating parent.id?

Because the foreign key is defined with ON UPDATE CASCADE, the child rows automatically update their foreign key to match the new parent key, as shown in step 3 of the execution_table.

What would happen if ON UPDATE SET NULL was used instead of CASCADE?

The child.parent_id would be set to NULL after the parent.id update, instead of being updated to the new value. This behavior differs from the CASCADE action shown in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of child.parent_id after step 2?

A 1

B 2

C NULL

D Undefined

💡 Hint

Check the 'Child.parent_id After' column in row for step 2.

At which step does the child.parent_id get updated to match the new parent.id?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the step where 'Child.parent_id After' changes from 1 to 2.

If the foreign key used ON UPDATE SET NULL, what would child.parent_id be after the update?

A 1

B NULL

C 2

D Unchanged

💡 Hint

Refer to key_moments explanation about ON UPDATE SET NULL behavior.

Concept Snapshot

Foreign key ON UPDATE behavior:
- When a parent key changes, child keys react based on ON UPDATE rule.
- ON UPDATE CASCADE: child keys update to new parent key.
- ON UPDATE SET NULL: child keys become NULL.
- Ensures referential integrity automatically.
- Defined in FOREIGN KEY constraint syntax.

Full Transcript

This visual execution shows how foreign key ON UPDATE behavior works in SQL. When a parent table's key is updated, the database checks the foreign key constraint on the child table. If ON UPDATE CASCADE is set, child rows update their foreign keys to match the new parent key, maintaining integrity. The execution table traces each step: initial state, parent update, child update, and commit. Key moments clarify why child keys change and what would happen with different ON UPDATE actions. The quiz tests understanding of these steps and effects. This helps beginners see how foreign keys keep data consistent when parent keys change.