The SET TRANSACTION ISOLATION LEVEL statement is used in SQL to define the isolation level for a transaction. The isolation level determines how transaction integrity is visible to other transactions and can impact both concurrency and consistency.
Here’s how you can use the statement to change the isolation level of a transaction:
Syntax:
SET TRANSACTION ISOLATION LEVEL { { READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE | SNAPSHOT } | TO <level> }
Steps to Change the Isolation Level:
1. Choose the Isolation Level:
Determine the appropriate isolation level you need for your transaction:
- READ UNCOMMITTED: Allows dirty reads; transactions can see uncommitted changes from other transactions.
- READ COMMITTED: Default level in many databases; transactions can only see changes that are committed.
- REPEATABLE READ: Ensures that if a row is read twice in the same transaction, the same values are returned, preventing non-repeatable reads.
- SERIALIZABLE: The highest isolation level; transactions are completely isolated from each other, preventing phantom reads.
- SNAPSHOT: Provides a view of the data as it was at the start of the transaction, allowing for consistency without locks.
2. Set the Isolation Level:
Execute the SET TRANSACTION ISOLATION LEVEL statement before starting your transaction. The following is an example using READ COMMITTED:
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
3. Begin Your Transaction:
After setting the isolation level, you can start your transaction as follows:
BEGIN TRANSACTION;
4. Perform Your Operations:
Execute your SQL statements (SELECT, INSERT, UPDATE, DELETE) within this transaction.
5. Commit or Rollback:
After you've completed your operations, either commit or roll back the transaction:
COMMIT; -- or
ROLLBACK;
Example:
Here’s a complete example demonstrating the use of SET TRANSACTION ISOLATION LEVEL:
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
BEGIN TRANSACTION;
SELECT * FROM Employees WHERE Department = 'Sales';
-- Some operations like INSERT, UPDATE, etc. can be performed here
COMMIT;
Notes:
- The isolation level remains in effect for the session (connection) until explicitly changed or until the session ends.
- Some databases might have specific settings or might require elevated permissions to change transaction isolation levels.
- Not all databases support all isolation levels; always verify the capabilities of your specific database system (e.g., SQL Server, PostgreSQL, MySQL, etc.).
By using SET TRANSACTION ISOLATION LEVEL, you can control how your transactions interact with one another, managing the trade-offs between performance and data integrity.