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What does the MERGE command do?

SQLJunior~1 min read
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The MERGE command in SQL is used to combine data from two tables - it lets you insert new records, update existing ones, and remove duplicates if needed, all at once. It's often called "UPSERT" (update + insert).

Syntax (general form):

sql
MERGE INTO Target_Table AS T USING Source AS S ON T.Key = S.Key WHEN MATCHED THEN UPDATE SET T.Field1 = S.Field1, T.Field2 = S.Field2 WHEN NOT MATCHED THEN INSERT (Field1, Field2) VALUES (S.Field1, S.Field2);

Example:

sql
MERGE INTO Employees AS E USING NewEmployees AS N ON E.ID = N.ID WHEN MATCHED THEN UPDATE SET E.Name = N.Name, E.DepartmentID = N.DepartmentID WHEN NOT MATCHED THEN INSERT (ID, Name, DepartmentID) VALUES (N.ID, N.Name, N.DepartmentID);

What it does

  1. If a record with that ID already exists in the Employees table → it updates the data (UPDATE).
  2. If the record doesn't exist → it adds a new one (INSERT).

Why it's used:

  • It's convenient for syncing tables, importing new data while updating old data at the same time.
  • It avoids the need for separate code to check whether a record exists before inserting or updating it.

Put simply, MERGE is a command for "update or insert" a record into a table, depending on whether it already exists.

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