View (database)
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In database theory, a view consists of a stored query accessible as a virtual table composed of the result set of a query. Unlike ordinary tables (base tables) in a relational database, a view does not form 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 subsequent invocations of the view.
Views can provide advantages over tables:
- Views can represent a subset of the data contained in a table
- Views can join and simplify multiple tables into a single virtual table
- Views can act as aggregated tables, where the database engine aggregates data (sum, average etc) and presents the calculated results 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 take very little space to store; the database contains only the definition of a view, not a copy of all the data it presents
- Depending on the SQL engine used, views can provide extra security
- Views can limit the degree of exposure of a table or tables to the outer world
Just as functions (in programming) can provide abstraction, so database users can create abstraction by using views. 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 normalization of databases above second normal form would become much more difficult. Views can make it easier to create lossless join decomposition.
Just as rows in a base table lack any defined ordering, rows available through a view do not appear with any default sorting. A view is a relational table, and the relational model defines a table as a set of rows. Since sets are not ordered - by definition - the rows in a view are not ordered, either. Therefore, an ORDER BY clause in the view definition is meaningless. The SQL standard (SQL:2003) does not allow an ORDER BY clause in a subselect in a CREATE VIEW statement, just as it is not allowed in a CREATE TABLE statement. However, sorted data can be obtained from a view, in the same way as any other table - as part of a query statement.
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[edit] Read-only vs. updatable views
Database practitioners can define views as read-only or updatable. If the database system can determine the reverse mapping from the view schema to the schema of the underlying base tables, then the view is updatable. INSERT, UPDATE, and DELETE operations can be performed on updatable views. Read-only views do not support such operations because the DBMS cannot map the changes to the underlying base tables.
Some systems support the definition of INSTEAD OF triggers on views. This technique allows the definition of other logic for execution in place of an insert, update, or delete operation on the views. Thus database systems can implement data modifications based on read-only views. However, an INSTEAD OF trigger does not change the read-only or updatable property of the view itself.
[edit] Advanced view features
Various database management systems have extended the views from read-only subsets of data.
The Oracle database introduced the concept of materialized views: pre-executed, non-virtual views commonly used in 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 or trigger mechanisms behind its updates. DB2 provides so-called "materialized query tables" (MQTs) for the same purpose. Microsoft SQL Server introduced in its 2000 version indexed views which only store a separate index from the table, but not the entire data.
[edit] Equivalence
A view is equivalent to its source query. When queries are run against views, the query is modified. For example, if there exists a view named Accounts_view with the content as follows:
accounts view:
-------------
SELECT name,
money_received,
money_sent,
(money_received - money_sent) AS balance,
address,
...
FROM table_customers c
JOIN accounts_table a
ON a.customerid = c.customer_id
then the application could simply run a simple query such as:
Sample query
------------
SELECT name,
balance
FROM accounts_view
The RDBMS then takes the simple query, replaces the equivalent view, then sends the following to the optimiser:
Preprocessed query:
------------------
SELECT name,
balance
FROM (SELECT name,
money_received,
money_sent,
(money_received - money_sent) AS balance,
address,
...
FROM table_customers c JOIN accounts_table a
ON a.customerid = c.customer_id )
From this point on the optimizer takes the query, removes unnecessary complexity (for example: it is not necessary to read the address, since the parent invocation does not make use of it) and then sends the query to the SQL engine for processing.
[edit] External links
- Views in Microsoft SQL Server 2005
- Views in MySQL
- Views in PostgreSQL
- Views in SQLite
- What are Materialized views?
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