Update VS Merge in SQL

SQL (Structured Query Language) is a standard programming language for relational databases. It is used to manage and manipulate data in a database. SQL provides several commands to update data in tables, such as INSERT, UPDATE, DELETE, and MERGE. In this blog post, we will discuss the difference between MERGE and UPDATE commands in SQL.

Update Command: The UPDATE command is used to modify existing data in a table. The syntax of the UPDATE command is as follows:

UPDATE table_name
SET column1 = value1, column2 = value2,...
WHERE condition;

In this command, we specify the name of the table we want to update. We then use the SET keyword to specify the columns we want to modify and their new values. We use the WHERE clause to specify which rows we want to update.

For example, suppose we have a table named Students with the following data:

If we want to update the age of Bob to 23, we can use the following SQL statement:

UPDATE Students
SET Age = 23
WHERE Name = 'Bob';

This will update the age of Bob to 23.

Merge Command: The MERGE command is used to perform an upsert operation on a table. An upsert operation means that if a row already exists, it will be updated. Otherwise, a new row will be inserted. The syntax of the MERGE command is as follows:

MERGE INTO table_name
USING select_statement
ON condition
WHEN MATCHED THEN
    UPDATE SET column1 = value1, column2 = value2,...
WHEN NOT MATCHED THEN
    INSERT (column1, column2,...) VALUES (value1, value2,...);

In this command, we specify the name of the table we want to update/insert data into. We then use a select statement to specify the data we want to update/insert. We use the ON clause to specify the condition for matching the rows. We use the WHEN MATCHED THEN clause to specify what to do when a row is matched, and we use the WHEN NOT MATCHED THEN clause to specify what to do when a row is not matched.

For example, suppose we have a table named Employees with the following data:

If we want to upsert the salary of Bob to 65000, we can use the following SQL statement:

MERGE INTO Employees
USING (SELECT * FROM Employees WHERE Name = 'Bob') AS E
ON (Employees.EmployeeID = E.EmployeeID)
WHEN MATCHED THEN
    UPDATE SET Salary = 65000
WHEN NOT MATCHED THEN
    INSERT (EmployeeID, Name, Age, Gender, Salary)
    VALUES (4, 'Bob', 22, 'M', 65000);

This will upsert the salary of Bob to 65000. If Bob’s row already exists, it will be updated. Otherwise, a new row will be inserted.

Difference between Merge and Update in SQL: The main difference between the MERGE and UPDATE statements is that the MERGE statement can be used to modify data in two tables, while the UPDATE statement can only modify data in a single table.

Another key difference is that the MERGE statement allows you to perform different actions based on whether a match is found or not, such as inserting new rows or updating existing rows. The UPDATE statement, on the other hand, can only update existing rows.

One advantage of the MERGE statement is that it can be more efficient than using separate INSERT and UPDATE statements, especially when dealing with large amounts of data. It also allows you to perform complex updates and merges in a single statement.

Conclusion: In summary, the MERGE and UPDATE statements are both useful tools for updating or merging data in SQL databases. The MERGE statement is more versatile, allowing you to update or merge data from two tables and perform different actions based on whether a match is found or not. The UPDATE statement, on the other hand, is simpler and more straightforward, and is used to update data in a single table based on a specified condition. Understanding the differences between these two statements can help you choose the best approach for your specific needs.