SQL Transactions and ACID Properties

1. What is a Transaction?

A transaction is a group of SQL statements that are executed together. If any part of the transaction fails, the entire transaction can be rolled back, ensuring that the database remains in a consistent state.

Transactions typically follow the following structure:

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START TRANSACTION;

-- SQL statements go here

COMMIT; -- or ROLLBACK in case of error

2. ACID Properties

The ACID properties are a set of principles that guarantee reliable processing of database transactions:

2.1 Atomicity

Atomicity ensures that all operations within a transaction are completed successfully. If any operation fails, the transaction is aborted, and all changes made during the transaction are rolled back.

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START TRANSACTION;

INSERT INTO accounts (id, balance) VALUES (1, 100);
UPDATE accounts SET balance = balance - 50 WHERE id = 1;

-- If the second operation fails, the entire transaction will not commit
ROLLBACK; -- all changes are reverted

2.2 Consistency

Consistency ensures that a transaction brings the database from one valid state to another valid state. All data integrity constraints must be satisfied before and after the transaction.

For example, if a transaction deducts funds from one account, it must ensure that the account has sufficient funds before proceeding.

2.3 Isolation

Isolation ensures that transactions are executed in isolation from one another. Even if transactions are executed concurrently, the outcome should be the same as if they were executed sequentially.

This is typically achieved through locking mechanisms.

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START TRANSACTION;

-- Transaction A
UPDATE accounts SET balance = balance - 100 WHERE id = 1;

-- Transaction B
UPDATE accounts SET balance = balance + 100 WHERE id = 2;

COMMIT; -- Each transaction is isolated from the other

2.4 Durability

Durability ensures that once a transaction is committed, its changes are permanent, even in the event of a system failure. The changes are stored in non-volatile memory.

For example, after a successful COMMIT, the changes should persist in the database:

START TRANSACTION;

INSERT INTO transactions (account_id, amount) VALUES (1, 100);
COMMIT; -- The insert is durable, even if the system crashes immediately afterward

3. Conclusion

Transactions and ACID properties play a crucial role in ensuring the integrity and reliability of database operations. Understanding how to use transactions effectively can help you maintain a robust and consistent database system.