SQL

Where it fits

SQL is the standard language for working with relational databases. It connects application code to tables, indexes, transactions, joins, and query planning.

Mental model

SQL describes what data you want, not exactly how to fetch it. The database engine decides an execution plan using indexes, statistics, and algorithms.

Example

A backend service might use SQL to fetch all paid invoices for one customer, join them with account data, and update a transactionally consistent balance.

Common mistakes

• Forgetting indexes on common query paths.
• Building SQL strings unsafely instead of using parameters.
• Assuming NoSQL means SQL is obsolete.

Related concepts

Relational database, join, index, transaction, query planner.