View (SQL): Difference between revisions

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{{Short description|Database stored query result set}}

{{UnreferencedMore citations needed|date=MarchDecember 20202023}}

In a [[database]], a '''view''' is the [[result set]] of a ''stored'' [[Query language|query]], whichthat canpresents bea queriedlimited inperspective of the samedatabase manner asto a persistent database collection objectuser.<ref name="mysql-views">{{cite web | url=https://dev.mysql.com/doc/refman/8.0/en/views.html | title=25.5 Using Views | author=<!--Not stated--> | date=2023-12-12 | website=MySQL | publisher=Oracle | access-date=2023-12-12 | quote=Views are stored queries that when invoked produce a result set. A view acts as a virtual table. | archive-url=httphttps://web.archive.org/web/20231123070115/http://dev.mysql.com:80/doc/refman/8.0/en/views.html | archive-date=2023-11-23 | url-status=live }}</ref> This pre-established query command is kept in the [[data dictionary]]. Unlike ordinary ''[[base table]]s'' in a [[relational database]], a view does not form part of the [[database design|physical schema]]: as a result set, it is a virtual table<ref name="mysql-views" /> computed or collated dynamically from data in the database when access to that view is requested. Changes applied to the data in a relevant ''underlying table'' are reflected in the data shown in subsequent invocations of the view.

Views can provide advantages over tables:

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| ___location = <!-- not stated -->

| publisher = Osborne/McGraw-Hill

| pages = 291-292291–292

| isbn = 0072118458

}}</ref>

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* Views can hide the complexity of data. For example, a view could appear as Sales2020 or Sales2021, transparently [[Partition (database)|partitioning]] the actual underlying table.

* Views take very little space to store; the database contains only the definition of a view, not a copy of all the data that it presents.

* StructuresViews structure data in a way that classes of users find natural and intuitive.<ref name="SQL-reference-groff-weinberg"/>

Just as a [[function (computing)|function]] (in programming) can provide [[Abstraction (computer science)|abstraction]], so can a database view. In another parallel with functions, database users can manipulate nested views, thus one view can aggregate data from other views. Without the use of views, the [[Database normalization|normalization]] of databases above [[second normal form]] would become much more difficult. Views can make it easier to create lossless join decomposition.

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==Materialized views==

{{detailsfurther|Materialized view}}

Various [[database management system]]s have extended the views from read-only subsets of [[data]], particularly [[materialized view]]s: pre-executed, non-virtual views commonly used in [[Data warehouse|data warehousing]]. They give a static snapshot of the data and may include data from remote sources. The accuracy of a materialized view depends on the frequency of trigger mechanisms behind its updates.

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{{Databases}}

==References==

{{Reflist}}

{{DEFAULTSORT:View (Database)}}