Adron Hall explains how different relational database management systems describe schemas:
From the viewpoint of someone familiar with the general idea of a schema, it can indeed seem unusual that databases like SQL Server, Oracle, MariaDB/MySQL, and PostgreSQL each interpret and implement schemas in slightly (or sometimes, vastly) different ways. While the core idea behind a schema as a structured container or namespace for database objects remains somewhat consistent, the exact nature, utility, and behavior of schemas vary across these systems.
Read on for an overview of these for four products, as well as what the ANSI standard indicates.