**Course Title:** SQLite Mastery: Lightweight Database Management **Section Title:** Transactions and Data Integrity **Topic:** Write queries to implement transactions and manage data consistency in a multi-step process.(Lab topic) ### Introduction In the previous topic, we explored the concept of transactions and how they ensure data integrity by managing multiple database operations as a single, atomic unit. In this lab, we will write queries to implement transactions and manage data consistency in a multi-step process using SQLite. ### Transaction Principles Before we dive into writing queries, let's review the key principles of transactions in SQLite: 1. Atomicity: A transaction is treated as a single unit, and either all or none of the operations in the transaction are applied. 2. Consistency: A transaction must ensure data consistency, meaning that the data remains in a valid state before and after the transaction. 3. Isolation: Transactions are isolated from each other, and one transaction cannot interfere with another. 4. Durability: A transaction must be durable, meaning that once it is committed, its effects are permanent and cannot be rolled back. ### Implementing Transactions in SQLite To implement a transaction in SQLite, we use the following steps: 1. **BEGIN TRANSACTION**: We start a new transaction using the `BEGIN TRANSACTION` statement. 2. **Execute Queries**: We execute multiple queries as part of the transaction. 3. **COMMIT**: If all queries execute successfully, we commit the transaction using the `COMMIT` statement. 4. **ROLLBACK**: If any query fails, we roll back the transaction using the `ROLLBACK` statement to restore the database to its original state. ### Example Transaction Suppose we have two tables, `accounts` and `transfers`, and we want to transfer money from one account to another. We can implement this as a transaction: ```sql -- Create tables CREATE TABLE accounts ( id INTEGER PRIMARY KEY, balance REAL NOT NULL ); CREATE TABLE transfers ( id INTEGER PRIMARY KEY, from_account INTEGER NOT NULL, to_account INTEGER NOT NULL, amount REAL NOT NULL, FOREIGN KEY (from_account) REFERENCES accounts (id), FOREIGN KEY (to_account) REFERENCES accounts (id) ); -- Insert accounts INSERT INTO accounts (id, balance) VALUES (1, 1000.0); INSERT INTO accounts (id, balance) VALUES (2, 500.0); -- Start transaction BEGIN TRANSACTION; -- Update account balances UPDATE accounts SET balance = balance - 200.0 WHERE id = 1; UPDATE accounts SET balance = balance + 200.0 WHERE id = 2; -- Insert transfer record INSERT INTO transfers (from_account, to_account, amount) VALUES (1, 2, 200.0); -- Commit transaction COMMIT; -- Check account balances SELECT * FROM accounts; ``` In this example, if any of the queries (updates or insert) fail, the transaction will be rolled back, and the account balances will remain unchanged. ### Managing Data Consistency To manage data consistency, we can use constraints, such as PRIMARY KEY, FOREIGN KEY, and UNIQUE constraints, to ensure that data is valid and consistent. For example, we can add a PRIMARY KEY constraint to the `transfers` table to ensure that each transfer record is unique: ```sql ALTER TABLE transfers ADD CONSTRAINT transfer_unique UNIQUE (from_account, to_account, amount); ``` We can also use triggers to enforce data consistency rules. For example, we can create a trigger to prevent an account balance from going below zero: ```sql CREATE TRIGGER account_balance_check BEFORE UPDATE ON accounts FOR EACH ROW WHEN NEW.balance < 0 BEGIN SELECT RAISE(ABORT, 'Account balance cannot be negative'); END; ``` ### Conclusion In this lab, we learned how to write queries to implement transactions and manage data consistency in a multi-step process using SQLite. We reviewed transaction principles and implemented a transaction to transfer money between accounts. We also discussed how to manage data consistency using constraints and triggers. To practice your understanding of transactions and data consistency, try implementing a transaction to manage inventory levels in a warehouse. Don't forget to test your code using different scenarios to ensure data consistency. If you have any questions or need help with implementing transactions, please let us know. Your feedback is valuable, and we appreciate any comments you may have. **Next Topic:** We will explore indexing and its impact on performance. For more information on indexing, you can refer to the official SQLite documentation: [https://www.sqlite.org/chrono.html](https://www.sqlite.org/chrono.html). Leave a comment below if you have any questions or need further clarification on any of the concepts.