INSERT ON CONFLICT (upsert) in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the INSERT ON CONFLICT clause do in PostgreSQL?

It allows you to try inserting a new row but if a conflict occurs (like a duplicate key), you can specify an alternative action such as updating the existing row instead of failing.

beginner

What keyword is used with INSERT ON CONFLICT to update existing rows when a conflict happens?

The keyword DO UPDATE is used to update the existing row when a conflict occurs during an insert.

beginner

Explain the purpose of the ON CONFLICT (column_name) DO NOTHING clause.

It tells PostgreSQL to skip the insert if a conflict happens on the specified column, leaving the existing row unchanged and avoiding an error.

intermediate

In the statement INSERT INTO users (id, name) VALUES (1, 'Alice') ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name;, what does EXCLUDED.name mean?

EXCLUDED.name refers to the value that was attempted to be inserted for the name column. It is used to update the existing row with the new value.

beginner

Why is INSERT ON CONFLICT called an "upsert" operation?

Because it combines "update" and "insert": it inserts a new row if no conflict exists, or updates the existing row if a conflict happens.

What happens if you use ON CONFLICT (id) DO NOTHING and a row with the same id already exists?

A The existing row is deleted.

B The existing row is updated.

C An error is thrown.

D The insert is skipped and no error occurs.

DO NOTHING skips the insert when a conflict happens, so no error and no change to existing row.

Which clause lets you update an existing row during an insert conflict?

A DO UPDATE

B DO NOTHING

C ON DUPLICATE KEY

D REPLACE

DO UPDATE specifies how to update the existing row when a conflict occurs.

In ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name, what does EXCLUDED represent?

A The row that caused the conflict

B A temporary table

C The new values attempted to insert

D The existing row in the table

EXCLUDED contains the values that were attempted to be inserted but caused the conflict.

Can INSERT ON CONFLICT be used to insert multiple rows at once?

A No, only single-row inserts are supported.

B Yes, it works with multi-row inserts.

C Only if you use a transaction.

D Only with a trigger.

PostgreSQL supports multi-row inserts with ON CONFLICT for each row.

What is the main benefit of using INSERT ON CONFLICT?

A It prevents duplicate rows and allows updating existing data in one command.

B It speeds up SELECT queries.

C It automatically creates indexes.

D It deletes conflicting rows.

It helps avoid errors from duplicates and lets you update existing rows in a single statement.

Describe how the INSERT ON CONFLICT clause works in PostgreSQL and give an example use case.

Think about inserting data that might already be there.

Explain the difference between DO NOTHING and DO UPDATE in the context of INSERT ON CONFLICT.

One ignores conflict, the other fixes it.

Practice

1. What does the INSERT ON CONFLICT clause do in PostgreSQL?

easy

A. It inserts a new row or updates existing row if a conflict occurs.

B. It deletes rows that conflict with the new data.

C. It only inserts new rows and ignores conflicts without any update.

D. It creates a new table if a conflict is found.

Solution

1. Step 1: Understand the purpose and behavior of INSERT ON CONFLICT

   This clause helps handle duplicate key conflicts during insert operations by allowing an update instead of error. It either updates the existing row or ignores the insert, preventing errors.

2. Final Answer:

   It inserts a new row or updates existing row if a conflict occurs. -> Option A

3. Quick Check:

   INSERT ON CONFLICT = Insert or update [OK]

Hint: Think: insert new or update existing on conflict [OK]

Common Mistakes:

• Confusing ON CONFLICT with DELETE
• Assuming it only inserts without update
• Thinking it creates new tables

2. Which of the following is the correct syntax to update a column price on conflict for a table products with unique id?

easy

A. INSERT INTO products (id, price) VALUES (1, 100) ON CONFLICT DO NOTHING;

B. INSERT INTO products (id, price) VALUES (1, 100) ON DUPLICATE KEY UPDATE price = price + 1;

C. INSERT INTO products (id, price) VALUES (1, 100) ON CONFLICT (id) DO UPDATE SET price = EXCLUDED.price;

D. INSERT INTO products (id, price) VALUES (1, 100) ON CONFLICT (id) DO DELETE;

Solution

1. Step 1: Check PostgreSQL syntax for upsert and identify correct usage of EXCLUDED

   PostgreSQL uses ON CONFLICT (column) DO UPDATE SET with EXCLUDED to refer to new values. INSERT INTO products (id, price) VALUES (1, 100) ON CONFLICT (id) DO UPDATE SET price = EXCLUDED.price; correctly uses EXCLUDED.price to update the price on conflict of id.

2. Final Answer:

   INSERT INTO products (id, price) VALUES (1, 100) ON CONFLICT (id) DO UPDATE SET price = EXCLUDED.price; -> Option C

3. Quick Check:

   Correct syntax uses ON CONFLICT (id) DO UPDATE SET with EXCLUDED [OK]

Hint: Use EXCLUDED.column to update on conflict [OK]

Common Mistakes:

• Using MySQL syntax ON DUPLICATE KEY
• Forgetting EXCLUDED keyword
• Using DO DELETE which is invalid

3. Given the table users(id INT PRIMARY KEY, name TEXT, age INT) with data:
(1, 'Alice', 30)
What will be the result of this query?

INSERT INTO users (id, name, age) VALUES (1, 'Alice', 31) ON CONFLICT (id) DO UPDATE SET age = EXCLUDED.age RETURNING *;

medium

A. [] (empty result)

B. [{id:1, name:'Alice', age:31}]

C. Error: duplicate key value violates unique constraint

D. [{id:1, name:'Alice', age:30}]

Solution

1. Step 1: Understand the existing data/conflict and apply ON CONFLICT DO UPDATE

   Row with id=1 exists with age=30, new insert tries age=31 causing conflict on id. It updates age to EXCLUDED.age (31) and returns the updated row.

2. Final Answer:

   [{id:1, name:'Alice', age:31}] -> Option B

3. Quick Check:

   ON CONFLICT updates age to 31 and returns updated row [OK]

Hint: ON CONFLICT updates existing row and returns it [OK]

Common Mistakes:

• Expecting error on duplicate key
• Thinking old age remains unchanged
• Assuming empty result on update

4. Identify the error in this query:

INSERT INTO inventory (product_id, quantity) VALUES (10, 5) ON CONFLICT (product_id) DO UPDATE quantity = quantity + EXCLUDED.quantity;

medium

A. VALUES clause cannot be used with ON CONFLICT.

B. EXCLUDED keyword is not allowed in DO UPDATE.

C. ON CONFLICT must be followed by DO NOTHING, not DO UPDATE.

D. Missing SET keyword before update expressions.

Solution

1. Step 1: Review ON CONFLICT DO UPDATE syntax and identify missing SET keyword

   The DO UPDATE clause requires SET keyword before specifying column updates. The query misses SET before 'quantity = quantity + EXCLUDED.quantity', causing syntax error.

2. Final Answer:

   Missing SET keyword before update expressions. -> Option D

3. Quick Check:

   DO UPDATE requires SET before column assignments [OK]

Hint: Remember DO UPDATE SET column = value [OK]

Common Mistakes:

• Omitting SET keyword in DO UPDATE
• Misusing EXCLUDED keyword
• Confusing DO NOTHING and DO UPDATE

5. You want to insert a new user into users(id, name, score) but if the id exists, update the score only if the new score is higher. Which query correctly implements this?

hard

A. INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO UPDATE SET score = EXCLUDED.score WHERE EXCLUDED.score > users.score;

B. INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO UPDATE SET score = users.score WHERE EXCLUDED.score > users.score;

C. INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO NOTHING WHERE EXCLUDED.score > users.score;

D. INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO UPDATE SET score = EXCLUDED.score;

Solution

1. Step 1: Understand conditional update on conflict and use WHERE clause in DO UPDATE

   You want to update score only if new score is greater than existing score. INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO UPDATE SET score = EXCLUDED.score WHERE EXCLUDED.score > users.score; uses WHERE EXCLUDED.score > users.score to conditionally update score correctly.

2. Final Answer:

   INSERT INTO users (id, name, score) VALUES (2, 'Bob', 50) ON CONFLICT (id) DO UPDATE SET score = EXCLUDED.score WHERE EXCLUDED.score > users.score; -> Option A

3. Quick Check:

   Conditional update uses WHERE clause in DO UPDATE [OK]

Hint: Use WHERE in DO UPDATE for conditional changes [OK]

Common Mistakes:

• Updating unconditionally without WHERE
• Using DO NOTHING with WHERE (invalid)
• Setting score to old value mistakenly