Demonstrating the READ UNCOMMITTED Isolation Level in MySQL

Demonstrate Read Uncommitted Isolation Level

Write a MySQL query to demonstrate the READ UNCOMMITTED isolation level by reading uncommitted data.

Solution:

-- Transaction 1: Starts a new transaction
START TRANSACTION;

-- Update the salaries of all employees in DepartmentID = 2 by increasing them by 10%
-- This change is not yet committed, meaning other transactions should not see this update unless their isolation level allows it.
UPDATE Employees SET Salary = Salary * 1.1 WHERE DepartmentID = 2;


-- Transaction 2: Set the isolation level to READ UNCOMMITTED
-- This means this transaction can read **uncommitted** changes from other transactions.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

-- Transaction 2: Start a new transaction
START TRANSACTION;

-- Select all employees from DepartmentID = 2
-- Since the isolation level is READ UNCOMMITTED, this transaction **may read the uncommitted salary updates**
-- made by Transaction 1, even though Transaction 1 has not committed the changes yet.
SELECT * FROM Employees WHERE DepartmentID = 2;

-- Commit Transaction 2
-- Although no data was modified in Transaction 2, it commits the read operation.
COMMIT;


-- Transaction 1: Rollback the transaction
-- Since the transaction is being rolled back, the salary updates in Transaction 1 are discarded.
-- This creates a **dirty read** issue in Transaction 2 because it may have read **temporary, uncommitted** salary values 
-- that were never officially saved in the database.
ROLLBACK;

Explanation:

• Purpose of the Query:
• The goal is to demonstrate how READ UNCOMMITTED allows reading uncommitted data.
• Key Components:
• SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED: Allows dirty reads.
• Why avoid READ UNCOMMITTED?:
• It can lead to inconsistent data if uncommitted changes are rolled back.
• Real-World Application:
• For example, in a reporting system, you might avoid READ UNCOMMITTED to ensure data accuracy.

For more Practice: Solve these Related Problems:

• Write a MySQL query to show how a transaction reads uncommitted data from another transaction, leading to dirty reads.
• Write a MySQL query where a financial report queries uncommitted sales data, resulting in inaccurate analytics.
• Write a MySQL query that highlights the risk of reading uncommitted balance updates in a banking system.
• Write a MySQL query that demonstrates the impact of the READ UNCOMMITTED isolation level when querying a frequently updated table.

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous MySQL Exercise: Rollback on Constraint Violation.
Next MySQL Exercise: Use Transactions to Ensure Consistency.