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 SQL engines have extended the views from read-only subsets of data. Oracle Corporation introduced the concept of materialised views; pre-executed, non-virtual views commonly used in data warehousing. They exist as a static snapshot of the data and may include data from remote sources. The accuracy of a materialised view depends on the frequency or trigger mechanisms behind its updates. The Microsoft equivalent of this, introduced in SQL Server 2000, is an indexed view.
See also
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