ON DUPLICATE KEY UPDATE in MySQL - Mini Project: Build & Apply

Using ON DUPLICATE KEY UPDATE in MySQL

📖 Scenario: You are managing a small online store's product inventory. You want to add new products to the database, but if a product with the same product_id already exists, you want to update its price and stock instead of adding a duplicate entry.

🎯 Goal: Build a MySQL query that inserts a new product into the products table. If the product_id already exists, update the price and stock fields for that product.

📋 What You'll Learn

Create a products table with columns product_id (primary key), name, price, and stock.

Insert a product with specific values for product_id, name, price, and stock.

Add a query that uses ON DUPLICATE KEY UPDATE to update price and stock if the product_id already exists.

Ensure the final query correctly handles both insert and update scenarios.

💡 Why This Matters

🌍 Real World

Managing product inventory in an online store where products may be added or updated frequently.

💼 Career

Database administrators and backend developers often use ON DUPLICATE KEY UPDATE to efficiently handle insert-or-update operations.

1

Create the products table

Create a table called products with these columns: product_id as an integer primary key, name as a VARCHAR(50), price as DECIMAL(10,2), and stock as an integer.

MySQL

-- Create the products table with the specified columns
-- Your code here

Hint

Use CREATE TABLE and define product_id as the primary key.

2

Insert a product

Write an INSERT INTO products statement to add a product with product_id 101, name 'Coffee Mug', price 7.99, and stock 100.

MySQL

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  stock INT
);

-- Insert the product with the specified values
-- Your code here

Hint

Use INSERT INTO products (columns) VALUES (values) with the exact values given.

3

Write the ON DUPLICATE KEY UPDATE query

Write an INSERT INTO products statement to add a product with product_id 101, name 'Coffee Mug', price 8.99, and stock 150. Use ON DUPLICATE KEY UPDATE to update the price and stock columns if the product_id already exists.

MySQL

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  stock INT
);

INSERT INTO products (product_id, name, price, stock) VALUES (101, 'Coffee Mug', 7.99, 100);

-- Insert or update the product with ON DUPLICATE KEY UPDATE
-- Your code here

Hint

Use ON DUPLICATE KEY UPDATE followed by price = VALUES(price) and stock = VALUES(stock).

4

Complete the query with correct syntax

Ensure the final query includes the INSERT INTO products statement with ON DUPLICATE KEY UPDATE that updates price and stock for product_id 101. The query should be syntactically correct and ready to run.

MySQL

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  stock INT
);

INSERT INTO products (product_id, name, price, stock) VALUES (101, 'Coffee Mug', 7.99, 100);

INSERT INTO products (product_id, name, price, stock) VALUES (101, 'Coffee Mug', 8.99, 150)
ON DUPLICATE KEY UPDATE price = VALUES(price), stock = VALUES(stock);

-- Finalize the query syntax if needed
-- Your code here

Hint

Check that the query ends with a semicolon and uses the correct syntax for ON DUPLICATE KEY UPDATE.

Practice

1. What does the ON DUPLICATE KEY UPDATE clause do in a MySQL INSERT statement?

easy

A. It deletes rows with duplicate keys before inserting new ones.

B. It updates existing rows if a duplicate key is found during insert.

C. It ignores the insert if a duplicate key exists.

D. It creates a new table when a duplicate key is found.

Solution

1. Step 1: Understand the purpose of ON DUPLICATE KEY UPDATE

   This clause allows an INSERT statement to update existing rows if a duplicate key (like a primary key) is found instead of causing an error.

2. Step 2: Compare options with this behavior

   Only It updates existing rows if a duplicate key is found during insert. correctly describes this behavior. The other options describe different actions that are not what this clause does.

3. Final Answer:

   It updates existing rows if a duplicate key is found during insert. -> Option B

4. Quick Check:

   ON DUPLICATE KEY UPDATE = update on duplicate [OK]

Hint: It updates rows when duplicate keys appear during insert [OK]

Common Mistakes:

• Thinking it deletes rows instead of updating
• Assuming it ignores duplicates silently
• Confusing it with creating new tables

2. Which of the following is the correct syntax to use ON DUPLICATE KEY UPDATE in MySQL?

easy

A. INSERT INTO table (id, val) VALUES (1, 'a') ON DUPLICATE KEY UPDATE val = 'b';

B. INSERT INTO table (id, val) VALUES (1, 'a') DUPLICATE KEY UPDATE val = 'b';

C. INSERT INTO table (id, val) VALUES (1, 'a') ON DUPLICATE UPDATE val = 'b';

D. INSERT INTO table (id, val) VALUES (1, 'a') ON DUPLICATE KEY SET val = 'b';

Solution

1. Step 1: Recall the correct syntax format

   The correct syntax is INSERT INTO ... VALUES (...) ON DUPLICATE KEY UPDATE column = value;

2. Step 2: Match options with correct syntax

   Only INSERT INTO table (id, val) VALUES (1, 'a') ON DUPLICATE KEY UPDATE val = 'b'; matches the exact syntax. The other options have incorrect keywords or missing parts.

3. Final Answer:

   INSERT INTO table (id, val) VALUES (1, 'a') ON DUPLICATE KEY UPDATE val = 'b'; -> Option A

4. Quick Check:

   Correct syntax includes 'ON DUPLICATE KEY UPDATE' [OK]

Hint: Remember full phrase: ON DUPLICATE KEY UPDATE [OK]

Common Mistakes:

• Omitting the word KEY
• Using UPDATE without ON DUPLICATE KEY
• Replacing UPDATE with SET

3. Given the table users(id INT PRIMARY KEY, name VARCHAR(20), points INT) with data:
(1, 'Alice', 10)
What will be the result after running?

INSERT INTO users (id, name, points) VALUES (1, 'Alice', 20) ON DUPLICATE KEY UPDATE points = points + 5;

medium

A. A new row with id=1 and points=20 is inserted.

B. The existing row with id=1 is updated to points=5.

C. The existing row with id=1 is updated to points=15.

D. The query causes a duplicate key error.

Solution

1. Step 1: Identify duplicate key and update action

   The id 1 already exists, so the ON DUPLICATE KEY UPDATE clause triggers. It updates points by adding 5 to the existing value.

2. Step 2: Calculate updated points value

   Existing points = 10, so new points = 10 + 5 = 15.

3. Final Answer:

   The existing row with id=1 is updated to points=15. -> Option C

4. Quick Check:

   points = 10 + 5 = 15 [OK]

Hint: Update adds to existing value on duplicate key [OK]

Common Mistakes:

• Thinking it inserts a new row instead of updating
• Assuming points become 20 instead of adding 5
• Expecting an error on duplicate key

4. What is wrong with this query?

INSERT INTO products (id, stock) VALUES (5, 10) ON DUPLICATE KEY UPDATE stock = stock + 1,;

medium

A. The VALUES keyword is not allowed with ON DUPLICATE KEY UPDATE.

B. The ON DUPLICATE KEY UPDATE clause is missing a column.

C. The stock column cannot be updated this way.

D. The comma before the semicolon causes a syntax error.

Solution

1. Step 1: Check syntax of ON DUPLICATE KEY UPDATE clause

   The clause ends with a comma before the semicolon, which is invalid syntax in SQL.

2. Step 2: Identify correct syntax rules

   There should be no trailing comma after the last update assignment.

3. Final Answer:

   The comma before the semicolon causes a syntax error. -> Option D

4. Quick Check:

   Trailing comma in UPDATE clause = syntax error [OK]

Hint: No trailing comma after last update assignment [OK]

Common Mistakes:

• Leaving a comma after last column update
• Confusing VALUES usage with ON DUPLICATE KEY UPDATE
• Thinking stock column can't be updated

5. You want to insert a new user or update their score only if the new score is higher than the existing one. Which query correctly uses ON DUPLICATE KEY UPDATE for this?

hard

A. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = IF(score < VALUES(score), VALUES(score), score);

B. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = VALUES(score);

C. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = score + VALUES(score);

D. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = score - VALUES(score);

Solution

1. Step 1: Understand the requirement

   We want to update the score only if the new score is greater than the existing score.

2. Step 2: Analyze each option's update logic

   INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = VALUES(score); always replaces the score, ignoring if it's higher or not. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = IF(score < VALUES(score), VALUES(score), score); uses IF to update only if new score is higher. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = score + VALUES(score); adds scores, which is not the requirement. INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = score - VALUES(score); subtracts scores, which is incorrect.

3. Final Answer:

   INSERT INTO users (id, score) VALUES (2, 50) ON DUPLICATE KEY UPDATE score = IF(score < VALUES(score), VALUES(score), score); -> Option A

4. Quick Check:

   Use IF to update only if new score is higher [OK]

Hint: Use IF(condition, new, old) to update conditionally [OK]

Common Mistakes:

• Always overwriting score without condition
• Adding scores instead of comparing
• Subtracting scores mistakenly