Originally, in database theory, a view is a read only virtual or logical table composed of the result set of a query. Unlike ordinary tables in a relational database, a view is not part of the physical schema: it is a dynamic, virtual table computed or collated from data in the database. Changing the data in a table alters the data shown in the view.
Views can provide advantages over tables;
- They can subset the data contained in a table
- They can join and simplify multiple tables into a single virtual view
- Views can act as aggregated tables, where aggregated data (sum, average etc.) are calculated and presented as part of the data
- Views can hide the complexity of data, for example a view could appear as Sales2000 or Sales2001, transparently partitioning the actual underlying table
- Views do not incur any extra storage overhead
- Depending on the sql engine used, views can provide extra security.
Various database management systems have extended the views from read-only subsets of data. The Oracle database introduced the concept of materialized views, which are pre-executed, non-virtual views commonly used in data warehousing. They are a static snapshot of the data and may include data from remote sources. The accuracy of a materialized view depends on the frequency or trigger mechanisms behind its updates. The equivalent of this in Microsoft's SQL Server, introduced in the 2000 version, is an indexed view.
See also
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