🔀 MySQL Upsert Logic – INSERT or UPDATE in One Command

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🧲 Introduction – Why Use Upsert in MySQL?

In MySQL, upsert logic allows you to insert a new row or update an existing one if a duplicate key is found. It’s commonly needed when:

• 🔄 Inserting new records with fallback update
• 🧮 Maintaining counters or aggregates
• 📦 Syncing data from external systems
• 🔐 Avoiding duplication while preserving integrity

🎯 In this guide, you’ll learn:

• How to implement upsert logic using INSERT ... ON DUPLICATE KEY UPDATE
• Use of REPLACE INTO and its caveats
• Practical examples and best practices

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🔄 1. INSERT … ON DUPLICATE KEY UPDATE

This is the preferred method of implementing upserts in MySQL.

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🔹 Syntax

INSERT INTO table (columns...)
VALUES (values...)
ON DUPLICATE KEY UPDATE column = value;

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🔹 Example – Insert or Update Product Stock

INSERT INTO inventory (product_id, quantity)
VALUES (101, 5)
ON DUPLICATE KEY UPDATE quantity = quantity + 5;

✅ If product_id = 101 exists, quantity increases by 5
✅ If not, a new row is inserted

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🔹 Requirements

• Table must have a PRIMARY KEY or UNIQUE KEY
• The ON DUPLICATE KEY UPDATE clause only triggers if there’s a duplicate violation

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🔁 2. REPLACE INTO – Alternative Upsert

This command first deletes the existing row and then inserts the new one.

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🔹 Syntax

REPLACE INTO table (columns...)
VALUES (values...);

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🔹 Example

REPLACE INTO users (id, name)
VALUES (1, 'Alice');

✅ If id = 1 exists, the row is deleted and reinserted

⚠️ Caution: Auto-increment, foreign keys, and triggers may behave unexpectedly.

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🔍 INSERT vs REPLACE vs UPSERT

Operation	Behavior
INSERT	Fails on duplicate key
REPLACE	Deletes existing row, then inserts new one
INSERT ... ON DUPLICATE KEY	Updates existing row without delete
MERGE	❌ Not available in MySQL (but supported in other RDBMS like SQL Server)

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🧪 Real-World Use Case – Track Login Timestamps

CREATE TABLE login_audit (
  user_id INT PRIMARY KEY,
  last_login DATETIME
);

INSERT INTO login_audit (user_id, last_login)
VALUES (123, NOW())
ON DUPLICATE KEY UPDATE last_login = NOW();

✅ Logs login time whether it’s the first login or a repeat.

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💡 Best Practices

✅ Tip	💡 Why It Matters
Use unique or primary keys for upserts	Determines when to update instead of insert
Avoid REPLACE with complex relationships	May trigger delete-side effects
Be explicit in update clause	Prevents unintended overwrites
Use VALUES(col) or NEW.col in updates	Reuse inserted values easily
Monitor auto-increment with REPLACE	REPLACE may generate new IDs due to DELETE-INSERT

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🛠️ Advanced Example – Maintain Aggregate Score

INSERT INTO leaderboard (user_id, score)
VALUES (7, 10)
ON DUPLICATE KEY UPDATE score = score + 10;

✅ Increases score if user exists, otherwise inserts new user.

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📘 Common Use Cases

Scenario	Upsert Strategy Used
Count email opens per user	ON DUPLICATE KEY UPDATE counter = counter + 1
Log user session tokens	REPLACE INTO sessions (...) VALUES (...)
Update cart item quantity	ON DUPLICATE KEY UPDATE quantity = quantity + X
Merge product feeds	INSERT ... ON DUPLICATE KEY UPDATE price = VALUES(price)

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📌 Summary – Recap & Next Steps

MySQL’s upsert logic using INSERT ... ON DUPLICATE KEY UPDATE and REPLACE INTO enables efficient conflict-free inserts and updates in a single command.

🔍 Key Takeaways

• Use INSERT ... ON DUPLICATE KEY UPDATE for safe, non-destructive upserts
• Use REPLACE INTO only when you’re okay with delete-then-insert behavior
• Ensure a UNIQUE or PRIMARY KEY exists on conflict columns
• Upsert logic reduces the need for complex SELECT before INSERT patterns

⚙️ Real-World Relevance

Used in user tracking, product syncing, queue systems, activity logging, and web applications with high-frequency inserts and updates.

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❓ FAQ – MySQL Upsert Logic

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❓ Does ON DUPLICATE KEY UPDATE require a unique key?

✅ Yes. It triggers only when a PRIMARY KEY or UNIQUE INDEX conflict occurs.

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❓ Is REPLACE INTO the same as UPSERT?

🚫 No. REPLACE INTO deletes and reinserts the row—it’s more like DELETE + INSERT.

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❓ Can I update multiple columns on conflict?

✅ Yes:

ON DUPLICATE KEY UPDATE col1 = VALUES(col1), col2 = VALUES(col2);

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❓ Can I use ON DUPLICATE KEY without a primary or unique key?

❌ No. Without a conflict condition, the UPDATE clause won’t trigger.

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❓ Is MERGE available in MySQL?

❌ No. MERGE is not part of MySQL. Use upsert logic instead.

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