ON DUPLICATE KEY UPDATE in MySQL

Introduction

This helps you add a new row or update an existing row if a duplicate key is found. It saves time by doing both in one command.

When you want to add a new user but update their info if they already exist.

When inserting sales data and updating the total if the record is already there.

When logging events and you want to increase a count if the event happened before.

When syncing data from another system and you want to avoid duplicate entries.

Syntax

MySQL

INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...) ON DUPLICATE KEY UPDATE column1 = value1, column2 = value2, ...;

The duplicate key is usually a PRIMARY KEY or UNIQUE index.

You can update one or more columns after the duplicate key is found.

Examples

If user with id=1 exists, update age to 30; otherwise, add new user.

MySQL

INSERT INTO users (id, name, age) VALUES (1, 'Alice', 30) ON DUPLICATE KEY UPDATE age = VALUES(age);

Insert product or update price if SKU already exists.

MySQL

INSERT INTO products (sku, price) VALUES ('A123', 10.99) ON DUPLICATE KEY UPDATE price = VALUES(price);

Add 100 to quantity if item_id 5 exists; otherwise, insert new row.

MySQL

INSERT INTO inventory (item_id, quantity) VALUES (5, 100) ON DUPLICATE KEY UPDATE quantity = quantity + VALUES(quantity);

Sample Program

This creates a users table, inserts Alice with age 25, then tries to insert Alice again with age 30. Because id=1 exists, it updates age to 30. Finally, it selects Alice's record.

MySQL

CREATE TABLE IF NOT EXISTS users (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

INSERT INTO users (id, name, age) VALUES (1, 'Alice', 25);

INSERT INTO users (id, name, age) VALUES (1, 'Alice', 30) ON DUPLICATE KEY UPDATE age = VALUES(age);

SELECT * FROM users WHERE id = 1;

Important Notes

Make sure the table has a PRIMARY KEY or UNIQUE index for this to work.

You can use VALUES(column) in the UPDATE part to refer to the new value.

This is a quick way to avoid separate SELECT checks before INSERT or UPDATE.

Summary

Use ON DUPLICATE KEY UPDATE to insert or update in one step.

It works when a duplicate key (like PRIMARY KEY) is found.

You can update one or more columns with new values or calculations.

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