Editing Database schema (section)

{{Short description|Visual representation of database system relationships}}
{{Use mdy dates|date=January 2019}}
{{Use American English|date=January 2019}}
[[File:MediaWiki 1.28.0 database schema.svg|thumb|upright=1.32|[[MediaWiki]] 1.28.0 database schema. Many [[FOSS]] software tools allow modelling of DB layout/schemes like this. Visual representation often may also be exported as a production-ready source code made in DB-compatible languages like [[SQL]].]]
The '''database schema''' is the structure of a [[database]] described in a [[formal language]] supported typically by a [[relational database management system]] (RDBMS). The term "[[wikt:schema|schema]]" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of [[relational database]]s). The formal definition of a [[database]] schema is a set of formulas (sentences) called [[integrity constraints]] imposed on a database.{{citation needed|date=January 2016}} 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]].<ref name="source1" /> The states of a created [[conceptual schema]] are transformed into an explicit mapping, the database schema. This describes how real-world entities are [[Data modeling|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-user]]s."<ref name="source3"/> 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 [[Data integrity#Types of integrity constraints|integrity constraints]] specifically for an application and the constraints specifically for a type of database, all expressed in the same database language.<ref name="source1" /> In a [[relational database]], the schema defines the [[Table (database)|tables]], [[Field (computer science)|fields]], [[Relational model|relationship]]s, [[View (database)|view]]s, [[Index (database)|index]]es, [[Software package (installation)|package]]s, [[stored procedure|procedure]]s, [[subroutine|function]]s, [[Queue (data structure)|queue]]s, [[Database trigger|trigger]]s, [[Data type|type]]s, [[sequence]]s, [[materialized view]]s, [[Synonym (database)|synonym]]s, database links, [[Directory (file systems)|directories]], [[XML schema]]s, and other elements.

A database 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. In other words, schema is the structure of the database that defines the objects in the database.

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