ON DUPLICATE KEY UPDATE in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does ON DUPLICATE KEY UPDATE do in MySQL?

It updates an existing row if a new insert would cause a duplicate key error, instead of causing an error.

beginner

Which SQL statement uses ON DUPLICATE KEY UPDATE?

The INSERT statement can use ON DUPLICATE KEY UPDATE to handle duplicate keys gracefully.

intermediate

How do you update a column named count by adding 1 when a duplicate key occurs?

Use ON DUPLICATE KEY UPDATE count = count + 1 in your INSERT statement.

intermediate

Can ON DUPLICATE KEY UPDATE be used to update multiple columns?

Yes, you can update multiple columns by separating assignments with commas, like ON DUPLICATE KEY UPDATE col1 = val1, col2 = val2.

beginner

What happens if no duplicate key is found when using ON DUPLICATE KEY UPDATE?

The row is inserted normally without any update.

What does ON DUPLICATE KEY UPDATE do in a MySQL INSERT statement?

A Deletes the existing row if a duplicate key is found

B Throws an error and stops the query

C Ignores the insert if a duplicate key is found

D Updates the existing row if a duplicate key is found

It updates the existing row instead of throwing an error when a duplicate key is found.

Which keyword is used with INSERT to handle duplicate keys?

A ON ERROR IGNORE

B ON DUPLICATE KEY UPDATE

C IF EXISTS UPDATE

D ON CONFLICT DO NOTHING

ON DUPLICATE KEY UPDATE is the MySQL syntax for handling duplicate keys during insert.

How do you increase a numeric column visits by 1 on duplicate key?

A ON DUPLICATE KEY UPDATE visits = visits + 1

B ON DUPLICATE KEY UPDATE visits = 1

C ON DUPLICATE KEY UPDATE visits += 1

D ON DUPLICATE KEY UPDATE visits++

You must explicitly set visits = visits + 1 in the update clause.

Can ON DUPLICATE KEY UPDATE update multiple columns?

A Yes, by separating assignments with commas

B No, only one column can be updated

C Yes, but only if columns are numeric

D No, it only inserts new rows

Multiple columns can be updated by separating each assignment with commas.

What happens if no duplicate key exists when using ON DUPLICATE KEY UPDATE?

A The row is updated anyway

B The query fails

C The row is inserted normally

D The query is ignored

If no duplicate key exists, the row is inserted as usual.

Explain how ON DUPLICATE KEY UPDATE works in MySQL and give a simple example.

Think about inserting a row that might already exist.

Describe a real-life scenario where ON DUPLICATE KEY UPDATE is useful.

Consider tracking website visits or inventory counts.

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