[Solved] Consider these tables: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);INSERT INTO... | SQL

SQL - Table Constraints

Consider these tables:

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
);
INSERT INTO parent VALUES (1);
INSERT INTO child VALUES (10, 1);
UPDATE parent SET id = 2 WHERE id = 1;

What will be the value of parent_id in the child table after the update?

ANULL

B2

C1

DUpdate will fail

Step-by-Step Solution

Solution:

Step 1: Analyze ON UPDATE CASCADE effect
The foreign key has ON UPDATE CASCADE, so child.parent_id updates when parent.id changes.
Step 2: Apply update to child table
Parent id changes from 1 to 2, so child.parent_id changes from 1 to 2 automatically.
Final Answer:
2 -> Option B
Quick Check:
ON UPDATE CASCADE updates child keys = 2 [OK]

Quick Trick: ON UPDATE CASCADE changes child keys on parent update [OK]

Common Mistakes:

MISTAKES

Expecting child key to remain 1
Thinking update fails
Assuming child key becomes NULL

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More SQL Quizzes

Consider these tables:

Step 1: Analyze ON UPDATE CASCADE effect

Step 2: Apply update to child table

Final Answer:

Quick Check:

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