Meet Rahul, a data engineer at an e-learning platform. Every night, he receives a spreadsheet from the marketing team with updates to customer data—some are new signups, some are updates to old customers, and a few need to be removed.

His challenge?

“How do I keep the master Customers table in the database in sync with this daily file—without running three separate queries?”

Chapter 1: The Old Way – Multiple Queries

Rahul used to write:

1. UPDATE existing records
2. INSERT new ones
3. DELETE obsolete ones

Each with its own logic and filters.

It worked, but was messy and error-prone.

Chapter 2: The Discovery – Enter MERGE

One day, his senior Dev said:

“Why not use the MERGE command?

It lets you update, insert, or delete in one go, based on matching conditions.”

Rahul tried it.

And it worked like magic.

Chapter 3: What MERGE Does (In Simple Words)

MERGE is like a smart negotiator between two tables (or datasets):

• The target: where you want to apply changes (e.g., Customers)
• The source: the incoming changes (e.g., UpdatedCustomerList)

It checks each row in the source and decides:

• If it matches a record in the target → UPDATE it
• If it doesn’t match → INSERT it as new
• If something in the target is missing from the source → optionally DELETE it

Chapter 4: Rahul’s New SQL Superpower

MERGE INTO Customers AS Target
USING UpdatedCustomerList AS Source
ON Target.CustomerID = Source.CustomerID

WHEN MATCHED THEN 
    UPDATE SET Target.Email = Source.Email, Target.Name = Source.Name

WHEN NOT MATCHED BY TARGET THEN 
    INSERT (CustomerID, Name, Email)
    VALUES (Source.CustomerID, Source.Name, Source.Email)

WHEN NOT MATCHED BY SOURCE THEN 
    DELETE;

In one clean command, Rahul could sync the tables.

Chapter 5: Why It Matters

• Faster development: fewer lines, less maintenance
• Cleaner logic: easier to understand and review
• Data consistency: fewer mistakes across INSERTs/UPDATEs
• Real-world need: syncing CRM systems, inventory lists, or user accounts

Conclusion:

Rahul no longer dreads the daily data sync.

He tells his team:

“MERGE is like hiring a smart assistant that looks at both lists and says,

‘I’ll update this, add that, and remove the rest—don’t worry.’”