SQL database

A SQL database stores data in tables of rows and columns with a defined structure, and relationships between those tables that the database itself enforces. SQL is the language used to query it. PostgreSQL, MySQL and SQL Server are the familiar examples.

Its defining property is that the database, not the application, guarantees consistency. A column that must not be empty cannot be empty; an order cannot reference a customer who does not exist; a transaction either completes fully or not at all. Every application touching the data inherits those guarantees for free.

This is the right default for almost anything with money, users or records in it. Modern relational databases also handle JSON documents, full-text search and vector similarity competently, which removes most of the historical reasons to reach for something else.