Les12[1]Creating Views

Copyright ‫س‬ Oracle Corporation, 1999. All rights reserved.
12
Creating Views

12-2 Copyright ‫س‬ Oracle Corporation, 1999. All rights reserved.
Objectives
After completing this lesson, you should
be able to do the following:
• Describe a view
• Create a view
• Retrieve data through a view
• Alter the definition of a view
• Insert, update, and delete data through
a view
• Drop a view

Objectives
After completing this lesson, you should
be able to do the following:
• Describe an inline view
• Perform “Top-N” Analysis

Database Objects
Description
Basic unit of storage; composed of rows
and columns
Logically represents subsets of data from
one or more tables
Generates primary key values
Improves the performance of some queries
Alternative name for an object
Object
Table
View
Sequence
Index
Synonym

What Is a View?
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
----- ------- --------- ----- --------- ----- ----- -------
7839 KING PRESIDENT 17-NOV-81 5000 10
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7900 JAMES CLERK 7698 03-DEC-81 950 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7369 SMITH CLERK 7902 17-DEC-80 800 20
7788 SCOTT ANALYST 7566 09-DEC-82 3000 20
7876 ADAMS CLERK 7788 12-JAN-83 1100 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
EMP Table
EMPNO ENAME JOB MGR HIREDATE SAL COMM
DEPTNO
----- -------- --------- ---- --------- ------ ----- ------
-
7839 KING PRESIDENT 17-NOV-81 5000
10
7782 CLARK MANAGER 7839 09-JUN-81 1500 300
10
7934 MILLER CLERK 7782 23-JAN-82 1300
10
7566 JONES MANAGER 7839 02-APR-81 2975
20
7788 SCOTT ANALYST 7566 09-DEC-82 3000
20
7876 ADAMS CLERK 7788 12-JAN-83 1100
20
7369 SMITH CLERK 7902 17-DEC-80 800
20
7902 FORD ANALYST 7566 03-DEC-81 3000
20
7698 BLAKE MANAGER 7839 01-MAY-81 2850
EMPNO ENAME JOB
------ -------- -----------
7839 KING PRESIDENT
7782 CLARK MANAGER
7934 MILLER CLERK
EMPVU10 View

Why Use Views?
• To restrict data access
• To make complex queries easy
• To allow data independence
• To present different views of the same
data

Simple Views
and Complex Views
Feature Simple Views Complex Views
Number of tables One One or more
Contain functions No Yes
Contain groups of data No Yes
DML through view Yes Not always

Creating a View
• You embed a subquery within the
CREATE VIEW statement.
• The subquery can contain complex
SELECT syntax.
• The subquery cannot contain an
ORDER BY clause.
CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view
[(alias[, alias]...)]
AS subquery
[WITH CHECK OPTION [CONSTRAINT constraint]]
[WITH READ ONLY];

Creating a View
• Create a view, EMPVU10, that contains
details of employees in department 10.
• Describe the structure of the view by
using the SQL*Plus DESCRIBE
command.
SQL> DESCRIBE empvu10
SQL> CREATE VIEW empvu10
2 AS SELECT empno, ename, job
3 FROM emp
4 WHERE deptno = 10;
View created.

Creating a View
• Create a view by using column aliases
in the subquery.
• Select the columns from this view by
the given alias names.
SQL> CREATE VIEW salvu30
2 AS SELECT empno EMPLOYEE_NUMBER, ename NAME,
3 sal SALARY
4 FROM emp
View created.

Retrieving Data from a View
EMPLOYEE_NUMBER NAME SALARY
--------------- ---------- ---------
7698 BLAKE 2850
7654 MARTIN 1250
7499 ALLEN 1600
7844 TURNER 1500
7900 JAMES 950
7521 WARD 1250
6 rows selected.
SQL> SELECT *
2 FROM salvu30;

Querying a View
USER_VIEWS
EMPVU10
SELECT empno, ename, job
FROM emp
WHERE deptno = 10;
SQL*Plus
SELECT *
FROM empvu10;
EMP
7839 KING PRESIDENT
7782 CLARK MANAGER
7934 MILLER CLERK

Modifying a View
• Modify the EMPVU10 view by using
CREATE OR REPLACE VIEW clause. Add
an alias for each column name.
• Column aliases in the CREATE VIEW
clause are listed in the same order as the
columns in the subquery.
SQL> CREATE OR REPLACE VIEW empvu10
2 (employee_number, employee_name, job_title)
4 FROM emp
View created.

Creating a Complex View
Create a complex view that contains group
functions to display values from two tables.
SQL> CREATE VIEW dept_sum_vu
2 (name, minsal, maxsal, avgsal)
3 AS SELECT d.dname, MIN(e.sal), MAX(e.sal),
4 AVG(e.sal)
5 FROM emp e, dept d
6 WHERE e.deptno = d.deptno
7 GROUP BY d.dname;
View created.

Rules for Performing
DML Operations on a View
• You can perform DML operations on simple
views.
• You cannot remove a row if the view
contains the following:
– Group functions
– A GROUP BY clause
– The DISTINCT keyword
– The pseudocolumn ROWNUM keyword

Rules for Performing
DML Operations on a View
• You cannot modify data in a view if it contains:
– Any of the conditions mentioned in the
previous slide
– Columns defined by expressions
– The ROWNUM pseudocolumn
• You cannot add data if:
– The view contains any of the conditions
mentioned above or in the previous slide
– There are NOT NULL columns in the base
tables that are not selected by the view

Using the WITH CHECK OPTION
Clause
• You can ensure that DML on the view stays
within the domain of the view by using the
WITH CHECK OPTION clause.
• Any attempt to change the department
number for any row in the view will fail
because it violates the WITH CHECK OPTION
constraint.
2 AS SELECT *
3 FROM emp
4 WHERE deptno = 20
5 WITH CHECK OPTION CONSTRAINT empvu20_ck;
View created.

Denying DML Operations
• You can ensure that no DML operations
occur by adding the WITH READ ONLY
option to your view definition.
2 (employee_number, employee_name, job_title)
4 FROM emp
5 WHERE deptno = 10
6 WITH READ ONLY;
View created.
• Any attempt to perform a DML on any
row in the view will result in Oracle
Server error.

Removing a View
Remove a view without losing data
because a view is based on underlying
tables in the database.
SQL> DROP VIEW empvu10;
View dropped.
DROP VIEW view;

Inline Views
• An inline view is a subquery with an
alias (correlation name) that you can
use within a SQL statement.
• An inline view is similar to using a
named subquery in the FROM clause of
the main query.
• An inline view is not a schema object.

“Top-N” Analysis
• Top-N queries ask for the n largest or
smallest values of a column.
– What are the ten best selling
products?
– What are the ten worst selling
products ?
• Both largest values and smallest values
sets are considered Top-N queries.

Performing “Top-N” Analysis
The high-level structure of a Top-N
analysis query is:
SQL> SELECT [column_list], ROWNUM
2 FROM (SELECT [column_list] FROM table
3 ORDER BY Top-N_column)
4 WHERE ROWNUM <= N

Example of “Top-N” Analysis
To display the top three earners names
and salaries from the EMP table.
SQL> SELECT
2 FROM (SELECT ename,sal FROM emp
3 ORDER BY sal DESC)
4 WHERE ROWNUM <= 3;
ROWNUM as RANK, sal
ename,
RANK
---------
1
2
3
ENAME
----------
KING
SCOTT
FORD
SAL
---------
5000
3000
3000

Summary
• A view is derived from data in other tables
or other views.
• A view provides the following
advantages:
– Restricts database access
– Simplifies queries
– Provides data independence
– Allows multiple views of the same data
– Can be dropped without removing the
underlying data

Summary
• An inline view is a subquery with an alias
name.
• “Top-N” analysis can be done using:
– Subquery
– Outer query

Practice Overview
• Creating a simple view
• Creating a complex view
• Creating a view with a check constraint
• Attempting to modify data in the view
• Displaying view definitions
• Removing views

Les12[1]Creating Views

More Related Content

What's hot (20)

Similar to Les12[1]Creating Views (20)

More from siavosh kaviani (14)

Recently uploaded (20)

Les12[1]Creating Views