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<title>Database schema</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Database schema</span></span>

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<p>The <b>database schema</b> is the structure of a <a href="Database" title="Database">database</a> described in a <a href="Formal_language" title="Formal language">formal language</a> supported typically by a <a href="Relational_database_management_system" class="mw-redirect" title="Relational database management system">relational database management system</a> (RDBMS). The term "<a href="https://en.wiktionary.org/wiki/schema" class="extiw external" title="wikt:schema">schema</a>" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of <a href="Relational_database" title="Relational database">relational databases</a>). The formal definition of a <a href="Database" title="Database">database</a> schema is a set of formulas (sentences) called <a href="Integrity_constraints" class="mw-redirect" title="Integrity constraints">integrity constraints</a> 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 <a href="Database_language" class="mw-redirect" title="Database language">database language</a>.<sup id="cite_ref-source1_1-0" class="reference"><a href="#cite_note-source1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The states of a created <a href="Conceptual_schema" title="Conceptual schema">conceptual schema</a> are transformed into an explicit mapping, the database schema. This describes how real-world entities are <a href="Data_modeling" title="Data modeling">modeled</a> in the database.

</p><p>"A database schema specifies, based on the <a href="Database_administrator" title="Database administrator">database administrator</a>'s knowledge of possible applications, the facts that can enter the database, or those of interest to the possible <a href="End-user" class="mw-redirect" title="End-user">end-users</a>."<sup id="cite_ref-source3_2-0" class="reference"><a href="#cite_note-source3-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> The notion of a database schema plays the same role as the notion of theory in <a href="Predicate_calculus" class="mw-redirect" title="Predicate calculus">predicate calculus</a>. A model of this "theory" closely corresponds to a database, which can be seen at any instant of time as a <a href="Mathematical_object" title="Mathematical object">mathematical object</a>. Thus a schema can contain formulas representing <a href="Data_integrity#Types_of_integrity_constraints" title="Data integrity">integrity constraints</a> specifically for an application and the constraints specifically for a type of database, all expressed in the same database language.<sup id="cite_ref-source1_1-1" class="reference"><a href="#cite_note-source1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> In a <a href="Relational_database" title="Relational database">relational database</a>, the schema defines the <a href="Table_(database)" title="Table (database)">tables</a>, <a href="Field_(computer_science)" title="Field (computer science)">fields</a>, <a href="Relational_model" title="Relational model">relationships</a>, <a href="View_(database)" class="mw-redirect" title="View (database)">views</a>, <a href="Index_(database)" class="mw-redirect" title="Index (database)">indexes</a>, <a href="Software_package_(installation)" class="mw-redirect" title="Software package (installation)">packages</a>, <a href="Stored_procedure" title="Stored procedure">procedures</a>, <a href="Subroutine" class="mw-redirect" title="Subroutine">functions</a>, <a href="Queue_(data_structure)" class="mw-redirect" title="Queue (data structure)">queues</a>, <a href="Database_trigger" title="Database trigger">triggers</a>, <a href="Data_type" title="Data type">types</a>, <a href="Sequence" title="Sequence">sequences</a>, <a href="Materialized_view" title="Materialized view">materialized views</a>, <a href="Synonym_(database)" title="Synonym (database)">synonyms</a>, database links, <a href="Directory_(file_systems)" class="mw-redirect" title="Directory (file systems)">directories</a>, <a href="XML_schema" title="XML schema">XML schemas</a>, and other elements.

</p><p>A database generally stores its schema in a <a href="Data_dictionary" title="Data dictionary">data dictionary</a>. 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.

</p><p>In an <a href="Oracle_Database" title="Oracle Database">Oracle Database</a> system, the term "schema" has a slightly different connotation.

</p>

<div class="mw-heading mw-heading2"><h2 id="Ideal_requirements_for_schema_integration">Ideal requirements for schema integration</h2></div>

<p>The requirements listed below influence the detailed structure of schemas that are produced. Certain applications will not require that all of these conditions are met, but these four requirements are the most ideal.

</p>

<dl><dt>Overlap preservation</dt>

<dd>Each of the overlapping elements specified in the input mapping is also in a database schema relation.<sup id="cite_ref-source2_3-0" class="reference"><a href="#cite_note-source2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></dd></dl>

<dl><dt>Extended overlap preservation</dt>

<dd>Source-specific elements that are associated with a source’s overlapping elements are passed through to the database schema.<sup id="cite_ref-source2_3-1" class="reference"><a href="#cite_note-source2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></dd></dl>

<dl><dt>Normalization</dt>

<dd>Independent entities and relationships in the source data should not be grouped together in the same relation in the database schema. In particular, source specific schema elements should not be grouped with overlapping schema elements, if the grouping co-locates independent entities or relationships.<sup id="cite_ref-source2_3-2" class="reference"><a href="#cite_note-source2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></dd></dl>

<dl><dt>Minimality</dt>

<dd>If any elements of the database schema are dropped then the database schema is not ideal.<sup id="cite_ref-source2_3-3" class="reference"><a href="#cite_note-source2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></dd></dl>

<div class="mw-heading mw-heading2"><h2 id="Example_of_two_schema_integrations">Example of two schema integrations</h2></div>

<p>Suppose we want a mediated schema to <a href="Data_integration" title="Data integration">integrate</a> two travel databases, Go-travel and Ok-flight.

</p><p><b><code>Go-travel</code></b> has two relations:

</p>

<div class="mw-highlight mw-highlight-lang-text mw-content-ltr" dir="ltr"><pre>Go-flight(flight-number, time, meal(yes/no))

Go-price(flight-number, date, price)

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<p><b><code>Ok-flight</code></b> has just one relation:

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<div class="mw-highlight mw-highlight-lang-text mw-content-ltr" dir="ltr"><pre>Ok-flight(flight-number, date, time, price, nonstop(yes/no))

</pre></div>

<p>The overlapping information in Go-travel’s and Ok-flight’s schemas could be represented in a mediated schema:<sup id="cite_ref-source2_3-4" class="reference"><a href="#cite_note-source2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-highlight mw-highlight-lang-text mw-content-ltr" dir="ltr"><pre>Flight(flight-number, date, time, price)

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Oracle_database_specificity">Oracle database specificity</h2></div>

<p>In the context of <a href="Oracle_Database" title="Oracle Database">Oracle Databases</a>, a <b>schema object</b> is a logical data storage structure.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

</p><p>An Oracle database associates a separate schema with each database <b>user</b>.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

A schema comprises a collection of schema objects. Examples of schema objects include:

</p>

<ul><li><a href="Table_(database)" title="Table (database)">tables</a></li>

<li><a href="View_(database)" class="mw-redirect" title="View (database)">views</a></li>

<li><a href="Sequence" title="Sequence">sequences</a></li>

<li><a href="Synonym_(database)" title="Synonym (database)">synonyms</a></li>

<li><a href="Index_(database)" class="mw-redirect" title="Index (database)">indexes</a></li>

<li>clusters</li>

<li>database links</li>

<li><a href="Snapshot_(computer_storage)" title="Snapshot (computer storage)">snapshots</a></li>

<li><a href="Stored_procedure" title="Stored procedure">procedures</a></li>

<li>functions</li>

<li>packages</li></ul>

<p>On the other hand, non-schema objects may include:<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p>

<ul><li>users</li>

<li>roles</li>

<li>contexts</li>

<li>directory objects</li></ul>

<p>Schema objects do not have a one-to-one correspondence to physical files on disk that store their information. However, <a href="Oracle_database" class="mw-redirect" title="Oracle database">Oracle databases</a> store schema objects logically within a <a href="Tablespace" title="Tablespace">tablespace</a> of the database. The data of each object is physically contained in one or more of the tablespace's <a href="Datafile" class="mw-redirect" title="Datafile">datafiles</a>. For some objects (such as tables, indexes, and clusters) a <a href="Database_administrator" title="Database administrator">database administrator</a> can specify how much disk space the Oracle <a href="RDBMS" class="mw-redirect" title="RDBMS">RDBMS</a> allocates for the object within the tablespace's datafiles.

</p><p>There is no necessary relationship between schemas and tablespaces: a tablespace can contain objects from different schemas, and the objects for a single schema can reside in different tablespaces. Oracle database specificity does, however, enforce platform recognition of nonhomogenized sequence differentials, which is considered a crucial limiting factor in virtualized applications.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading2"><h2 id="Microsoft_SQL_Server">Microsoft SQL Server</h2></div>

<p>In <a href="Microsoft_SQL_Server" title="Microsoft SQL Server">Microsoft SQL Server</a>, the default schema of every database is the dbo schema.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup>

</p>

<ul><li><a href="Data_element" title="Data element">Data element</a></li>

<li><a href="Data_mapping" title="Data mapping">Data mapping</a></li>

<li><a href="Data_model" title="Data model">Data model</a></li>

<li><a href="Database_design" title="Database design">Database design</a></li>

<li><a href="Database_model" title="Database model">Database model</a></li>

<li><a href="Entity%E2%80%93relationship_model" title="Entity–relationship model">Entity–relationship model</a></li>

<li><a href="Knowledge_representation_and_reasoning" title="Knowledge representation and reasoning">Knowledge representation and reasoning</a></li>

<li><a href="Object-role_modeling" class="mw-redirect" title="Object-role modeling">Object-role modeling</a></li>

<li><a href="Schema_matching" title="Schema matching">Schema matching</a></li>

<li><a href="Three-schema_approach" title="Three-schema approach">Three-schema approach</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFAshdownKyte2010" class="citation book cs1">Ashdown, Lance; Kyte, Tom (February 2010). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20100129054802/http://download.oracle.com/docs/cd/E11882_01/server.112/e10713/tablecls.htm#CNCPT111"><i>Oracle Database Concepts 11g Release 2 (11.2)</i></a>. <i>et al</i>. Oracle Corporation. Archived from <a rel="nofollow" class="external text" href="http://download.oracle.com/docs/cd/E11882_01/server.112/e10713/tablecls.htm#CNCPT111">the original</a> on January 29, 2010<span class="reference-accessdate">. Retrieved <span class="nowrap">April 14,</span> 2010</span>. <q>A database schema is a logical container for data structures, called schema objects. Examples of schema objects are tables and indexes.</q></cite></span>

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<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><cite id="CITEREFVanMSFT2024" class="citation web cs1">VanMSFT (May 9, 2024). <a rel="nofollow" class="external text" href="https://learn.microsoft.com/en-us/sql/relational-databases/security/authentication-access/ownership-and-user-schema-separation?view=sql-server-ver16">"Ownership and user-schema separation in SQL Server - SQL Server"</a>. <i>learn.microsoft.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">October 31,</span> 2024</span>.</cite></span>

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</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://weblogs.asp.net/scottgu/Tip_2F00_Trick_3A00_-Online-Database-Schema-Samples-Library">Tip/Trick: Online Database Schema Samples Library</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20081217074637/http://msdn.microsoft.com/en-us/library/bb187299%28SQL.80%29.aspx">Database Schema Samples</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080828210315/http://ciobriefings.com/Publications/WhitePapers/DesigningtheStarSchemaDatabase/tabid/101/Default.aspx">Designing the Star Schema Database</a></li></ul>

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</style><div id="Database414" style="font-size:114%;margin:0 4em"><a href="Database" title="Database">Database</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Main</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database#Major_database_usage_requirements" title="Database">Requirements</a></li>

<li><a href="Database_theory" title="Database theory">Theory</a></li>

<li><a href="Database_object" title="Database object">Database object</a></li>

<li><a href="Database_model" title="Database model">Models</a></li>

<li><a href="Database_management_system" class="mw-redirect" title="Database management system">Database management system</a></li>

<li><a href="Database_machine" title="Database machine">Machine</a></li>

<li><a href="Database_engine" title="Database engine">Engine</a></li>

<li><a href="Database_server" title="Database server">Server</a></li>

<li><a href="Database_application" title="Database application">Application</a></li>

<li><a href="Database_connection" title="Database connection">Connection</a>

<ul><li><a href="Datasource" title="Datasource">datasource</a></li>

<li><a href="Database_administrator" title="Database administrator">Administrator</a></li>

<li><a href="Synonym_(database)" title="Synonym (database)">Synonym</a></li>

<li>Types</li>

<li><a href="Comparison_of_database_tools" class="mw-redirect" title="Comparison of database tools">Tools</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database#Database_languages" title="Database">Languages</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Data_definition_language" title="Data definition language">Data definition</a></li>

<li><a href="Data_manipulation_language" title="Data manipulation language">Data manipulation</a></li>

<li><a href="Query_language" title="Query language">Query</a>

<ul><li><a href="Information_retrieval_query_language" class="mw-redirect" title="Information retrieval query language">information retrieval</a></li></ul></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database_security" title="Database security">Security</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_activity_monitoring" title="Database activity monitoring">Activity monitoring</a></li>

<li><a href="Database_audit" title="Database audit">Audit</a></li>

<li><a href="Database_forensics" title="Database forensics">Forensics</a></li>

<li><a href="Negative_database" title="Negative database">Negative database</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database_design" title="Database design">Design</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Entity%E2%80%93relationship_model" title="Entity–relationship model">Entities and relationships</a> (and <a href="Enhanced_entity%E2%80%93relationship_model" title="Enhanced entity–relationship model">Enhanced</a> notation)</li>

<li><a href="Database_normalization" title="Database normalization">Normalization</a></li>

<li><a href="Database_refactoring" title="Database refactoring">Refactoring</a></li>

<li><a href="Cardinality_(data_modeling)" title="Cardinality (data modeling)">Cardinality</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Programming</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_abstraction_layer" title="Database abstraction layer">Abstraction layer</a></li>

<li><a href="Object%E2%80%93relational_mapping" title="Object–relational mapping">Object–relational mapping</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Management</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_virtualization" title="Database virtualization">Virtualization</a></li>

<li><a href="Database_tuning" title="Database tuning">Tuning</a>

<ul><li><a href="Database_caching" title="Database caching">caching</a></li></ul></li>

<li><a href="Data_migration#Database_migration" title="Data migration">Migration</a></li>

<li><a href="Database_preservation" title="Database preservation">Preservation</a></li>

<li><a href="Database_integrity" class="mw-redirect" title="Database integrity">Integrity</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Lists_of_databases" title="Lists of databases">Lists of</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="List_of_academic_databases_and_search_engines" title="List of academic databases and search engines">Academic</a></li>

<li><a href="List_of_biological_databases" title="List of biological databases">Biological</a></li>

<li><a href="List_of_biodiversity_databases" title="List of biodiversity databases">Biodiversity</a></li>

<li><a href="List_of_facial_expression_databases" title="List of facial expression databases">Facial expression</a></li>

<li><a href="List_of_online_databases" title="List of online databases">Online</a></li>

<li><a href="List_of_online_music_databases" title="List of online music databases">Online music</a></li>

<li><a href="List_of_online_real_estate_databases" title="List of online real estate databases">Online real estate</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database-centric_architecture" title="Database-centric architecture">Database-centric architecture</a></li>

<li><a href="Intelligent_database" title="Intelligent database">Intelligent database</a></li>

<li><a href="Two-phase_locking" title="Two-phase locking">Two-phase locking</a></li>

<li><a href="Locks_with_ordered_sharing" title="Locks with ordered sharing">Locks with ordered sharing</a></li>

<li><a href="Database_publishing" title="Database publishing">Publishing</a></li>

<li><a href="Halloween_Problem" title="Halloween Problem">Halloween Problem</a></li>

<li><a href="Log_shipping" title="Log shipping">Log shipping</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div><b><span class="noviewer" typeof="mw:File"><span title="WikiProject"></span></span> WikiProject</b> <b><span class="noviewer" typeof="mw:File"><span title="Category"></span></span> Category</b></div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-05-16" href="https://en.wikipedia.org/wiki/?title=Database_schema&oldid=1290660892">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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