Transaction management is essential in MySQL to ensure data consistency and integrity, especially in multi-user environments.

1. What is a Transaction?

A transaction is a set of SQL operations that are executed as a single unit. If one operation fails, the entire transaction is rolled back (it’s like undo) to maintain data consistency.

Example: Imagine you are transferring money from Alice’s account to Bob’s account. The transaction includes two steps:

1. Deducting money from Alice’s account
2. Adding money to Bob’s account

If step 1 succeeds but step 2 fails, the system should cancel the entire operation to avoid losing money.

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE name = 'Alice';
UPDATE accounts SET balance = balance + 100 WHERE name = 'Bob';
COMMIT; -- Saves changes permanently

If an error occurs before COMMIT, you can undo changes using:

ROLLBACK; -- Cancels the transaction

2. ACID Properties (Why Transactions Matter)

Transactions follow ACID principles:

• Atomicity: Either all steps succeed, or none do.
• Consistency: The database remains valid before and after transactions.
• Isolation: Transactions don’t interfere with each other.
• Durability: Once committed, changes are permanent.

3. Transaction Commands in MySQL

Here are key commands:

• START TRANSACTION; – Begins a transaction.
• COMMIT; – Saves changes permanently.
• ROLLBACK; – Reverts changes if something goes wrong.
• SAVEPOINT X; – Marks a point to which you can later rollback.
• ROLLBACK TO X; – Reverts only to a savepoint, not the entire transaction.

Example:

START TRANSACTION;
UPDATE inventory SET stock = stock - 1 WHERE product_id = 10;
SAVEPOINT halfway;
UPDATE orders SET status = 'Processed' WHERE order_id = 123;
-- If something goes wrong:
ROLLBACK TO halfway; -- Undo only the order update
COMMIT; -- Finalize the remaining changes

4. Transaction Isolation Levels

MySQL offers different isolation levels to manage concurrent transactions:

• READ UNCOMMITTED: Transactions see uncommitted changes (Risky, may show incorrect data).
• READ COMMITTED: Only committed changes are visible.
• REPEATABLE READ (Default): Ensures consistent data during a transaction.
• SERIALIZABLE: Highest level, prevents concurrent writes (slower but safest).

Example: To set the isolation level before starting a transaction:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
START TRANSACTION;
SELECT balance FROM accounts WHERE name = 'Alice';

5. Best Practices for Transactions

• Keep transactions short: Long-running transactions lock resources, affecting performance.
• Use COMMIT wisely: Don’t commit too early before verifying success.
• Handle errors properly: Always include ROLLBACK in case of failure.
• Monitor deadlocks: Avoid circular dependencies in concurrent transactions.

6. Conclusion

Transaction management is crucial for maintaining data accuracy in MySQL. By understanding ACID properties, commands, isolation levels, and best practices, you can ensure your database operations are reliable and efficient.