Demonstrating the REPEATABLE READ Isolation Level in SQL

Demonstrate Isolation Level REPEATABLE READ

Write a SQL query to demonstrate the REPEATABLE READ isolation level.

Solution:

-- Set the isolation level to REPEATABLE READ.
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

BEGIN TRANSACTION; -- Begin the transaction.

-- Read data from the Employees table.
SELECT * FROM Employees WHERE EmployeeID = 1;

-- Simulate another transaction attempting to modify the same data.
UPDATE Employees SET Salary = 55000 WHERE EmployeeID = 1;

COMMIT TRANSACTION; -- Commit the transaction.

Explanation:

1. Purpose of the Query :

1. The goal is to demonstrate how the REPEATABLE READ isolation level prevents non-repeatable reads by locking the selected rows.

2. Key Components :

1. SET TRANSACTION ISOLATION LEVEL REPEATABLE READ : Sets the isolation level.
2. SELECT and UPDATE : Demonstrates reading and modifying data within the transaction.

3. Why Use REPEATABLE READ?

1. This isolation level ensures that once a transaction reads data, no other transaction can modify it until the first transaction completes.

4. Real-World Application :

1. For example, in inventory management, you might use this isolation level to ensure that stock levels remain consistent during a transaction.

Additional Notes:

• The REPEATABLE READ isolation level prevents non-repeatable reads but allows phantom reads.
• Scenarios where REPEATABLE READ is appropriate include:
• Situations where data must remain consistent during a transaction.
• Preventing other transactions from modifying read data.
• Important Considerations :
• Be aware of potential locking issues that can arise with this isolation level.

For more Practice: Solve these Related Problems:

• Write a SQL query to demonstrate how the REPEATABLE READ isolation level prevents non-repeatable reads in a database.
• Write a SQL query to simulate two concurrent transactions where one reads data multiple times under the REPEATABLE READ isolation level.
• Write a SQL query to test whether modifications by other transactions are visible during repeated reads under the REPEATABLE READ isolation level.
• Write a SQL query to observe the locking behavior of the REPEATABLE READ isolation level when reading and updating the same rows.

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