A Database is an organized collection of structured information, or data, typically stored electronically in a computer system.

Think of an Address Book:

1. You can write names and numbers.
2. You can find someone's number by their name.
3. You can delete someone you don't talk to anymore.
4. The Database is just a giant, super-fast, digital version of this that can handle millions of names at the same time and never loses a single page.

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CRUD Operations

The four basic functions of persistent storage:

• Create: Adding new data.
• Read: Fetching data.
• Update: Modifying existing data.
• Delete: Removing data.

Transactions and ACID

In system design, we often need multiple operations to succeed together. E.g., Transferring money from Account A to Account B. ACID properties ensure data reliability:

1. Atomicity: All or nothing. Both accounts update, or neither does.
2. Consistency: Data follows all rules (e.g., no negative balances).
3. Isolation: Transactions don't interfere with each other.
4. Durability: Once saved, data stays saved even if the power goes out.

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Relational vs. Non-Relational

• Relational (SQL): Data is organized into tables with rows and columns (e.g., PostgreSQL, MySQL). Great for complex queries and strict data structure.
• Non-Relational (NoSQL): Data is organized as documents, key-value pairs, or graphs (e.g., MongoDB, Cassandra). Great for massive scale and flexible data.

Performance Indexing

A database index is like the Index at the back of a book. It allows the database to find data without having to scan every single row (Full Table Scan).

Common Questions

• "What is a primary key vs a foreign key?"
• "Explain the difference between a Database and a Cache."
• "What happens to a transaction if the server crashes mid-way?" 🛢️