Session #, Speaker Name Views 1/2/2019

Views A view is a virtual table defined by a query. It provides a mechanism to create alternate ways of working with the data in a database. A view acts much like a table. We can query it with a SELECT, and some views even allow INSERT, UPDATE, and DELETE. A view doesn’t have any data. All of its data are ultimately derived from tables (called base tables). Views are similar to derived tables.

Create a View We can create a view using the CREATE VIEW command. CREATE VIEW <view name> [(<column list>)] AS <SELECT statement>

Create Views

Query Views

View may contain expressions

Views The view is just a virtual table, any changes to the base tables are instantly reflected in the view data.

Why Views Usability- we can use a view as a wrapper around very complex SELECT statements to make our system more usable. Security- if we need to restrict the data a user can access, we can create a view containing only the permitted data. The user is given access to the view instead of the base table(s). Reduced Dependency- the database schema evolves over time as our enterprise changes. Such changes can break existing applications that expect a certain set of tables with certain columns. We can fix this by having our applications access views rather than base tables. When the base tables change, existing applications still work as long as the views are correct.

Updating Views We can even perform INSERT, UPDATE, and DELETE on a view, which is propagated to the underlying tables; however, there are restrictions on the kinds of views that can be updated. Unfortunately, not all DBMSs allow updating through views. Even if your DBMS allows updates to views, you should be careful about using it, because some results may be unexpected.

Drop Views DROP VIEW <view name> [CASCADE | RESTRICT] This removes the specified view; however, it does not change any of the data in the database. SQL does not allow a view to be dropped if view is contained in the SELECT statement of another view. This is the default behavior or the result of using the RESTRICT option. To remove such a view, specify the CASCADE option. This will cause any dependent views to be removed before the view is dropped. According to the SQL specification, you must specify either CASCADE or RESTRICT with DROP VIEW. It is not optional. We present it as optional because most (if not all) DBMSs treat it as optional, with a default behavior of RESTRICT. For compliance with the SQL standard, it’s a good idea to specify either CASCADE or RESTRICT.

Summary Views are virtual tables whose columns and data are defined by a SELECT statement. You may SELECT from views just like any other table. Some views even allow INSERT, UPDATE, and DELETE. Updates to the base tables that a view is built from are immediately reflected in the view.

Practice

Session #, Speaker Name Questions? 1/2/2019