Database schema

A database schema (/ˈski.mə/ skee-mə) of a database system is its structure described in a formal language supported by the database management system (DBMS).

Description

Schema is the structure within the database system that defines the objects in the database.

It is the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases).

The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database.

These integrity constraints ensure compatibility between parts of the schema.

All constraints are expressible in the same language.

A database can be considered a structure in realization of the database language.

The states of a created conceptual schema are transformed into an explicit mapping, the database schema.

The schema describes how real-world entities are modeled in the database.

"A database schema specifies, based on the database administrator's knowledge of possible applications, the facts that can enter the database, or those of interest to the possible end-users."

Predicate calculus

The notion of a database schema plays the same role as the notion of theory in predicate calculus.

A model of this "theory" closely corresponds to a database, which can be seen at any instant of time as a mathematical object.

Thus a schema can contain formulas representing integrity constraints specifically for an application and the constraints specifically for a type of database, all expressed in the same database language.

Relational databases

In a relational database, the schema defines the tables, fields, relationships, views, indexes, packages, procedures, functions, queues, triggers, types, sequences, materialized views, synonyms, database links, directories, XML schemas, and other elements.

Data dictionary

A databases generally stores its schema in a data dictionary.

Although a schema is defined in text database language, the term is often used to refer to a graphical depiction of the database structure.

Oracle

In an Oracle Database system, the term "schema" has a slightly different connotation.

See also

• Database
• Database management system
• Database system
• Formal language
• Integrity constraint
• Relational database

External links

• Database schema @ Wikipedia