Chapter 12
Data Manipulation Language
(DML)
View
• Objectives
•
•
•
•
•
Dr. Chitsaz
Definition
Create a view
Retrieve data from a view
Insert, delete, update to/from views
Drop a view
1
Views
Database Objects
1-TABLE: Physical unite
2-VIEW: Logical representation
3-SEQUENCE
4-SYNONYM
5-INDEX
2
Views
VIEW:
-What is a view? Logical representations
(Subset of data from one or more tables or views)
-Why use a view?
Restrict database access.
Make complex queries easy.
Represent data from different tables.
Represent different Views of the same
data.
3
Views
Creating a View:
CREATE [OR REPLACE] [FORCE | NOFORCE]
VIEW name AS
Subquery
[WITH CHECK OPTION [CONSTRAINT name]]
[WITH READ ONLY];
Use OR REPLACE if view already exists.
Use FORCE if the base tables are not existed (view will be created even if the base table
does not exists)
Use WITH CHECK OPTION: only rows that accessible by view can be updated.
Use WITH READ ONLY for select only
4
Views
CREATE VIEW
COSCStudent
AS
SELECT
ID, Name, GPA
FROM
Student
WHERE
Major=‘COSC’;
DESCRIBE COSCStudent;
5
Results
Name
Null?
Type
----------------------------------------------------ID
NOT NULL NUMBER(6)
NAME
VARCHAR2(80)
GPA
NUMBER(3,2)
6
Views
Aliases Column Name:
CREATE VIEW COSCStudent AS
SELECT ID
COSCid,
name COSCName,
GPA
FROM
Student
WHERE
Major=‘COSC’;
7
Views
Retrieving Data from View:
SELECT
*
FROM
COSCStudent;
SELECT
FROM
COSCid, COSCname
COSCStudent;
8
Results
COSCID COSCNAME
GPA
-----------------------------------------------243
James
3.21
102234
John
3.32
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Views
Modifying a View:
CREATE OR REPLACE VIEW COSCStudent
(Field1, Field2) //alias names for Major and Minor
AS
SELECT
Major, Minor
FROM
student
WHERE
major =‘COSC’;
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Views
Example:
CREATE VIEW COSCData
(minsal, maxsal, avgsal)
AS
SELECT
MIN(Salary), MAX (Salary),
AVG (Salary)
FROM
Faculty
WHERE
dept =‘COSC’;
11
SELECT
FROM
*
COSCData;
MINSAL MAXSAL AVGSAL
--------------------------------------------------20000
45000
33800
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Views
Modifying a View (continued):
CREATE VIEW
studentgrade AS
SELECT
Name, ID, c_num, grade
FROM
student, student_course;
CREATE VIEW majors
AS
SELECT
major, count (*) total
FROM
student
GROUP BY
major;
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SELECT *
FROM
majors;
MAJOR TOTAL
------------------------COSC
2
ENGL
1
MATH
4
14
Views
CREATE FORCE VIEW COSCStudent
AS
SELECT
ID, Name, GPA
FROM
NewStudent
WHERE
Major=‘COSC’;
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Views
Check Option:
CREATE VIEW
SELECT
COSCStudent AS
ID COSCid,
name COSCName, GPA
FROM
Student
WHERE
Major=‘COSC’
WITH CHECK OPTION
CONSTRAINT cosc_ck;
//You can only update the COSC students records.
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Views
Read Only Option:
CREATE VIEW
SELECT
COSCStudent AS
ID COSCid,
name COSCName, GPA
FROM
Student
WHERE
Major=‘COSC’
WITH READ ONLY;
//Data may not be modified
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Views
Removing a View:
DROP VIEW majors;
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User Views
• You can check data dictionary to check for
name of view and definition using
USER_VIEWS
• SELECT
*
• FROM
USER_VIEWS
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Rules for Performing DML operation
on a View:
• Simple view: you can perform DML
operation on simple view (not complex;
droved from more than one table)
• Can NOT REMOVE a row if view
contains:
– Group function (aggregated functions including null)
– GROUP BY clause (or having)
– DISTINCT clause
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Rules for Performing DML operation
on a View:
•
Can NOT MODIFY data in a view if it
contains:
1. The Following:
• Group function
• GROUP BY clause
• DISTINCT clause
2. Column defined by expression The
ROWNUM pseudocolumn
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Rules for Performing DML operation
on a View:
•
Can NOT ADD data in a view if it
contains:
1. The Following:
•
•
•
Group function
GROUP BY clause
DISTINCT clause
2. Column defined by expression
3. NOT NULL columns in the tables that are not
selected by view
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Views
Insert Data Into A View:
INSERT INTO
COSCStudent
(COSCid, COSCName, GPA)
VALUES
(1121,’SANDY’,3.33);
SELECT
FROM
WHERE
COSCid, COSCName, GPA
COSCStudent
Major=‘COSC’;
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Viewing constraints:
SELECT
constraint_name, constraint_type,
search_condition, status,last_change
FROM
user_constraints
WHERE
table_name=‘STUDENT’;
Result:
CONSTRAINT_NAME C STATUS LAST_CHANGE
--------------------------------------------------------------------SYS_C0098772
P ENABLED 18-NOV-05
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Viewing constraints:
SELECT
FROM
WHERE
constraint_name, column_name, owner
user_cons_columns
table_name=‘STUDENT’;
CONSTRAINT_NAME COLUMN_NAME OWNER
------------------------------------------------------------------SYS_C0098772
ID
CS640M2
This is list of columns that can be updated
USER_UPDATABLE_COLUMNS
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Materialized Views
To improve the performance of an application, you can make local
copies of remote tables.
Definition: Stored local copies of tables
Master table: remote site
Local Table: local site
Refresh interval: how frequently update the tables
You need to have Creating Materialized View
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Privilege to create materialized view
Materialized Views
CREATE TABLE local student
AS
SELECT *
From
student@Remote_connect;
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Materialized Views
Creating a Materialized View:
CREATE MATERIALIZED VIEW name
[REFRESH clause ]
AS
Subquery;
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Materialized Views
Creating a Materialized View:
Example:
CREATE MATERIALIZED VIEW localstudents
REFRESH FAST
START WITH SYSDATE NEXT SYSDATE+10
WITH PRIMARY KEY
AS
SELECT
*
FROM
student@remote_connect;
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