Why INSERT ON CONFLICT (upsert) in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if you could add or update data with one simple command and never worry about duplicates again?

The Scenario

Imagine you have a guest list for a party in a notebook. You want to add new guests, but if someone is already on the list, you want to update their details instead of writing them twice.

The Problem

Manually checking if a guest is already on the list before adding or updating takes a lot of time and can cause mistakes like duplicates or missing updates.

The Solution

INSERT ON CONFLICT lets you add new data or update existing data in one simple step, so you never have duplicates and always keep your list accurate.

Before vs After

✗ Before

DO $$ BEGIN IF EXISTS (SELECT 1 FROM guests WHERE id = 1) THEN UPDATE guests SET name = 'Alice' WHERE id = 1; ELSE INSERT INTO guests (id, name) VALUES (1, 'Alice'); END IF; END $$;

✓ After

INSERT INTO guests (id, name) VALUES (1, 'Alice') ON CONFLICT (id) DO UPDATE SET name = EXCLUDED.name;

What It Enables

This makes managing data fast, safe, and simple, even when many changes happen at once.

Real Life Example

When a website registers users, it can add new users or update existing user info without errors or duplicates using INSERT ON CONFLICT.

Key Takeaways

Manually checking for existing data is slow and error-prone.

INSERT ON CONFLICT combines insert and update in one command.

It keeps data clean and operations efficient.

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