A transaction is a group of SQL statements that execute as a single unit. Either all statements succeed (COMMIT) or all fail (ROLLBACK). Transactions ensure data consistency and prevent partial updates.

**Transaction Basics:**

- BEGIN: Start a transaction

- COMMIT: Save all changes permanently

- ROLLBACK: Undo all changes since BEGIN

**ACID Properties:**

- **Atomicity**: All or nothing (no partial updates)

- **Consistency**: Data remains valid (constraints enforced)

- **Isolation**: Transactions don't interfere with each other

- **Durability**: Committed changes are permanent

**Why Use Transactions:**

1. **Data Integrity**: Ensure related changes happen together

2. **Error Recovery**: Rollback on failure

3. **Consistency**: Maintain valid state

4. **Safety**: Test changes before committing

**Transaction Flow:**

1. BEGIN TRANSACTION

2. Execute SQL statements (INSERT, UPDATE, DELETE)

3. If all succeed: COMMIT

4. If any fail: ROLLBACK

**Auto-commit Mode:**

- By default, each statement is a transaction

- Explicit transactions override auto-commit

- Use BEGIN to start multi-statement transaction

Transactions are fundamental to data integrity. Every financial system, e-commerce platform, and critical application uses transactions. Understanding transactions prevents data corruption, ensures consistency, and enables safe error handling. Essential for backend developers and DBAs.

A bank transfer moves $100 from Account A to Account B. This requires two operations: (1) Subtract $100 from A, (2) Add $100 to B. If operation 1 succeeds but 2 fails, money disappears! Transactions ensure both operations succeed together or both fail - no partial updates.