Databases in System Design: SQL, NoSQL, and ACID Explained

Every application needs memory.

An e-commerce website must remember products and orders. A banking application must remember transactions. A social media platform must remember posts, likes, and comments.

This information is stored in databases.

Relational Databases (SQL)

Relational databases store information in structured tables and are queried using SQL.

ACID Properties

Atomicity

A transaction either succeeds completely or fails completely.

Consistency

The database always moves from one valid state to another.

Isolation

Transactions do not interfere with one another.

Durability

Committed data survives failures.

NoSQL Databases

Document databases, graph databases, and key-value stores provide flexibility and scalability.

SQL vs NoSQL

Use SQL when correctness is critical. Use NoSQL when flexibility and scale are priorities.