The parent_id column is NOT NULL, so it cannot be set to NULL.

Solution:Step 1: Check column nullabilityON DELETE SET NULL requires the foreign key column to allow NULL values.Step 2: Identify the causeIf parent_id is NOT NULL, the database cannot set it to NULL, so it leaves it unchanged.Final Answer:The parent_id column is NOT NULL, so it cannot be set to NUL

[Solved] You wrote this SQL:ALTER TABLE child ADD CONSTRAINT fk_parent FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE SET NULL;But when you delete a parent row, the child row's parent_id does not... | SQL

SQL - Table Constraints

You wrote this SQL:

ALTER TABLE child ADD CONSTRAINT fk_parent FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE SET NULL;

But when you delete a parent row, the child row's parent_id does not become NULL. What is the likely problem?

AThe foreign key syntax is incorrect and ignored by the database.

BON DELETE SET NULL only works with CASCADE.

CThe <code>parent_id</code> column is NOT NULL, so it cannot be set to NULL.

DYou must delete child rows manually before deleting parent.

Step-by-Step Solution

Solution:

Step 1: Check column nullability
ON DELETE SET NULL requires the foreign key column to allow NULL values.
Step 2: Identify the cause
If parent_id is NOT NULL, the database cannot set it to NULL, so it leaves it unchanged.
Final Answer:
The parent_id column is NOT NULL, so it cannot be set to NULL. -> Option C
Quick Check:
SET NULL needs nullable foreign key column [OK]

Quick Trick: Foreign key must allow NULL for SET NULL to work [OK]

Common Mistakes:

MISTAKES

Assuming syntax error causes issue
Thinking SET NULL requires CASCADE
Believing manual delete is always needed

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You wrote this SQL:

Step 1: Check column nullability

Step 2: Identify the cause

Final Answer:

Quick Check:

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