What is view ?
View is the simply subset of table which are stored logically in a database means a view is a virtual table in the database whose
contents are defined by a query.
To the database user, the view appears just
like a real table, with a set of named columns and rows of data. SQL creates the
illusion of the view by giving the view a name like a table name and storing the
definition of the view in the database.
Views are used for security purpose in databases,views restricts the user from viewing certain column and rows means by using view we can apply the restriction on accessing the particular rows and columns for specific user. Views
display only those
data which are mentioned in the query, so it shows only data which is
returned by the query that is defined at the time of creation of the View.
Advantages of views
Security
Each user can be given permission to access the database only through a small
set of views that contain the specific data the user is authorized to see, thus
restricting the user's access to stored data
Query Simplicity
A view can draw data from several different tables and present it as a single
table, turning multi-table queries into single-table queries against the view.
Structural simplicity
Views can give a user a "personalized" view of the database structure,
presenting the database as a set of virtual tables that make sense for that
user.
Consistency
A view can present a consistent, unchanged image of the structure of the
database, even if the underlying source tables are split, restructured, or
renamed.
Data Integrity
If data is accessed and entered through a view, the DBMS can automatically check
the data to ensure that it meets the specified integrity constraints.
Logical data independence.
View can make the application and database
tables to a certain extent independent. If there is no view, the
application must be based on a table. With the view, the program can be
established in view of above, to view the program with a database table
to be separated.
Disadvantages of views
Performance
Views create the appearance of a table, but the DBMS must still translate
queries against the view into queries against the underlying source tables. If
the view is defined by a complex, multi-table query then simple queries on the
views may take considerable time.
Update restrictions
When a user tries to update rows of a view, the DBMS must translate the request
into an update on rows of the underlying source tables. This is possible for
simple views, but more complex views are often restricted to read-only.