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.
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