Database/Schema

• Database Schema

  • Overview

    • A database schema describes the organization and rules of data in a database. In a relational database it defines tables, columns, data types, keys, relationships, and constraints. It is the database-specific form of a Schema.

  • Levels of description

    • A conceptual model names the things a domain cares about and how they relate, without choosing tables or storage structures.
    • A logical schema expresses those things in a data model, such as relational tables and foreign keys. A physical design chooses indexes, partitions, and other storage structures that affect performance.

  • Schema in use

    • A database uses schema constraints to reject invalid writes. Applications use the schema to form queries and interpret results. Some systems enforce a declared schema when data is written; others allow flexible records and apply shape expectations when data is read.
    • DDL expresses schema definitions and changes in SQL. A schema migration carries those changes into an existing database and may also transform stored data.

  • Related idea

    • Database/System/Theory studies how schemas interact with queries, transactions, storage, and recovery. A schema describes the structure of the data; the records are instances of that structure.