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Chapter 12 Data Manipulation Language

(DML)View

Dr. Chitsaz• Objectives

• Definition

• Create a view

• Retrieve data from a view

• Insert, delete, update to/from views

• Drop a view

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ViewsDatabase Objects

1-TABLE: Physical unite2-VIEW: Logical representation3-SEQUENCE4-SYNONYM5-INDEX

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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.

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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

Views

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CREATE VIEW COSCStudent AS

SELECT ID, Name, GPAFROM StudentWHERE Major=‘COSC’;

DESCRIBE COSCStudent;

Views

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Results

Name Null? Type

-----------------------------------------------------

ID NOT NULL NUMBER(6)

NAME VARCHAR2(80)

GPA NUMBER(3,2)

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Aliases Column Name:CREATE VIEW COSCStudent AS

SELECT ID COSCid, name COSCName,GPA

FROM StudentWHERE Major=‘COSC’;

Views

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Retrieving Data from View:

SELECT *

FROM COSCStudent;

SELECT COSCid, COSCname

FROM COSCStudent;

Views

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Results

COSCID COSCNAME GPA

------------------------------------------------

243 James 3.21

102234 John 3.32

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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’;

Views

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Example:

CREATE VIEW COSCData

(minsal, maxsal, avgsal)

AS

SELECT MIN(Salary), MAX (Salary),

AVG (Salary)

FROM Faculty

WHERE dept =‘COSC’;

Views

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SELECT *

FROM COSCData;

MINSAL MAXSAL AVGSAL

---------------------------------------------------

20000 45000 33800

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Modifying a View (continued):CREATE VIEW studentgrade AS

SELECT Name, ID, c_num, gradeFROM student, student_course;

CREATE VIEW majors ASSELECT major, count (*) totalFROM studentGROUP BY major;

Views

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SELECT *FROM majors;

MAJOR TOTAL-------------------------COSC 2ENGL 1MATH 4

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CREATE FORCE VIEW COSCStudent

AS

SELECT ID, Name, GPA

FROM NewStudent

WHERE Major=‘COSC’;

Views

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Check Option:CREATE VIEW COSCStudent AS

SELECT ID COSCid, name COSCName, GPA

FROM StudentWHERE Major=‘COSC’

WITH CHECK OPTION CONSTRAINT cosc_ck;

//You can only update the COSC students records.

Views

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Read Only Option:CREATE VIEW COSCStudent AS

SELECT ID COSCid, name COSCName, GPA

FROM StudentWHERE Major=‘COSC’

WITH READ ONLY;//Data may not be modified

Views

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Removing a View:

DROP VIEW majors;

Views

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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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Insert Data Into A View:INSERT INTO COSCStudent

(COSCid, COSCName, GPA)VALUES (1121,’SANDY’,3.33);

SELECT COSCid, COSCName, GPAFROM COSCStudentWHERE Major=‘COSC’;

Views

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Viewing constraints:

SELECT constraint_name, constraint_type, search_condition, status,last_change

FROM user_constraintsWHERE table_name=‘STUDENT’;

Result:CONSTRAINT_NAME C STATUS

LAST_CHANGE---------------------------------------------------------------------SYS_C0098772 P ENABLED 18-NOV-05

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Viewing constraints:

SELECT constraint_name, column_name, owner

FROM user_cons_columns

WHERE table_name=‘STUDENT’;

CONSTRAINT_NAME COLUMN_NAME OWNER-------------------------------------------------------------------SYS_C0098772 ID CS640M2

This is list of columns that can be updatedUSER_UPDATABLE_COLUMNS

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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 Privilege to create materialized view

Materialized Views

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CREATE TABLE local studentAS

SELECT *From student@Remote_connect;

Materialized Views

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Creating a Materialized View:

CREATE MATERIALIZED VIEW name [REFRESH clause ]

ASSubquery;

Materialized Views

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Creating a Materialized View:Example:

CREATE MATERIALIZED VIEW localstudentsREFRESH FASTSTART WITH SYSDATE NEXT SYSDATE+10WITH PRIMARY KEY

ASSELECT *FROM student@remote_connect;

Materialized Views