**Course Title:** SQLite Mastery: Lightweight Database Management **Section Title:** Transactions and Data Integrity **Topic:** Understanding transactions and SQLite's ACID properties. **Introduction** In the previous topics, we learned about creating and managing databases, performing queries, and designing a well-structured database schema. However, ensuring data integrity and consistency is crucial when working with databases. Transactions are a fundamental concept in database systems that enable multiple operations to be executed as a single, all-or-nothing unit of work. In this topic, we will delve into the world of transactions and explore SQLite's ACID properties. **What are transactions?** A transaction is a sequence of operations that are executed together as a single unit of work. Transactions ensure that either all operations are successful, or none of them are, maintaining the consistency and integrity of the data. This means that if any part of the transaction fails, the entire transaction is rolled back, and the database returns to its previous state. **ACID properties** ACID (Atomicity, Consistency, Isolation, and Durability) is a set of properties that ensure the reliability and integrity of database transactions. These properties were first introduced by Andreas Reuter and Theo Härder in 1983. 1. **Atomicity**: This property ensures that either all operations within a transaction are executed successfully, or none of them are. If any part of the transaction fails, the entire transaction is rolled back. 2. **Consistency**: This property ensures that the database remains in a consistent state before and after a transaction. SQLite checks the data against predefined constraints, such as primary keys and foreign keys, to ensure data integrity. 3. **Isolation**: This property ensures that multiple transactions can be executed concurrently without interference. SQLite ensures that the results of one transaction do not affect the results of another transaction. 4. **Durability**: This property ensures that once a transaction is committed, its effects are permanent and cannot be rolled back. **SQLite's transaction support** SQLite provides full support for transactions, adhering to the ACID properties. SQLite uses a journaling mechanism to ensure atomicity and durability. In case of a failure, the journal can be used to roll back the transaction. **How transactions work in SQLite** When a transaction is started using the `BEGIN TRANSACTION` command, SQLite creates a journal file to record the changes made during the transaction. If the transaction is committed successfully using the `COMMIT` command, the changes are written to the database file, and the journal file is removed. If the transaction fails or is rolled back using the `ROLLBACK` command, the journal file is used to restore the database to its previous state. **Real-world example** Suppose we have a bank database with two tables: `accounts` and `transactions`. We want to transfer money from one account to another using a transaction. ```sql -- Initialize the accounts table CREATE TABLE accounts ( id INTEGER PRIMARY KEY, balance REAL ); -- Initialize the transactions table CREATE TABLE transactions ( id INTEGER PRIMARY KEY, from_account INTEGER, to_account INTEGER, amount REAL ); -- Insert some data into the accounts table INSERT INTO accounts (id, balance) VALUES (1, 1000.0); INSERT INTO accounts (id, balance) VALUES (2, 500.0); -- Start a transaction BEGIN TRANSACTION; -- Debit the from_account UPDATE accounts SET balance = balance - 200.0 WHERE id = 1; -- Credit the to_account UPDATE accounts SET balance = balance + 200.0 WHERE id = 2; -- Insert a transaction record INSERT INTO transactions (id, from_account, to_account, amount) VALUES (1, 1, 2, 200.0); -- Commit the transaction COMMIT; ``` **Practical takeaway** In this example, we started a transaction using the `BEGIN TRANSACTION` command. We then performed two updates and an insert operation, and finally committed the transaction using the `COMMIT` command. If any of these operations fail, the entire transaction will be rolled back, ensuring data integrity and consistency. **What's next?** In the next topic, we will explore the commands used to manage transactions in SQLite, including `BEGIN TRANSACTION`, `COMMIT`, and `ROLLBACK`. We will learn how to use these commands to create, manage, and commit transactions. For further learning, we recommend checking out the official SQLite documentation on transactions: [Transaction Control Language (TCL)](https://www.sqlite.org/lang_transaction.html). We encourage you to practice and experiment with transactions using the SQLite command-line tool or a GUI client like DB Browser for SQLite. If you have any questions or need help, feel free to leave a comment below.