MySQL is the most popular open-source relational database, known for speed, reliability, and ease of use. Key features: (1) InnoDB storage engine: ACID compliance, transactions, foreign keys, row-level locking. (2) JSON support: Native JSON type (5.7+), JSON functions, generated columns for indexing. (3) Replication: Master-slave for read scaling, binary log, GTID for failover. (4) AUTO_INCREMENT: Automatic ID generation, simple and fast. (5) Full-Text Search: FULLTEXT indexes, MATCH() AGAINST(). Real-world: Facebook uses MySQL for billions of queries/day. YouTube uses MySQL for video metadata. Trade-off: Simpler than PostgreSQL (easier to learn) but less powerful (no arrays, limited JSON).

**1. Storage Engines - InnoDB vs MyISAM:**

MySQL supports multiple storage engines. InnoDB is default since MySQL 5.5.

**InnoDB (Default):**

- **ACID compliance**: Transactions with COMMIT/ROLLBACK

- **Foreign keys**: Referential integrity

- **Row-level locking**: Better concurrency

- **Crash recovery**: Automatic recovery from crashes

- **Use for**: Transactional data (orders, payments, users)

**MyISAM (Legacy):**

- **No transactions**: No COMMIT/ROLLBACK

- **No foreign keys**: No referential integrity

- **Table-level locking**: Poor concurrency

- **Fast reads**: Faster than InnoDB for read-only

- **Use for**: Read-only data (logs, archives)

**Comparison:**

```sql

-- Create InnoDB table (default)

CREATE TABLE orders (

order_id INT AUTO_INCREMENT PRIMARY KEY,

user_id INT,

total DECIMAL(10, 2),

FOREIGN KEY (user_id) REFERENCES users(user_id)

) ENGINE=InnoDB;

-- Supports transactions, foreign keys

-- Create MyISAM table

CREATE TABLE logs (

log_id INT AUTO_INCREMENT PRIMARY KEY,

message TEXT,

created_at TIMESTAMP

) ENGINE=MyISAM;

-- No transactions, no foreign keys, fast reads

-- Transaction example (InnoDB only)

START TRANSACTION;

INSERT INTO orders (user_id, total) VALUES (1, 99.99);

INSERT INTO order_items (order_id, product_id) VALUES (LAST_INSERT_ID(), 123);

COMMIT;

-- Both inserts succeed or both fail (atomic)

-- MyISAM: No transactions

INSERT INTO logs (message) VALUES ('User logged in');