B.Com Computer Notes: May 2021

VIEW, SEQUENCE & SYNONYMS in RDBMS

View

A view is a logical entity also called virtual table which is created from the base table or main table.

The main objective of creating a view is to provide secure access over the main table. In some cases when admin want to show some column from the specific table, admin or database programmer can do this by creating separate table i.e. virtual table or VIEW with the same data for each user as per the requirement but Data Redundancy is the limitation of this approach.

Read-only view:

A view by which user can only view records but unable to insert, update or delete records is called read only view.

Updatable view:

A view by which user can perform DML statements like select, insert, update or delete operation is called updatable view.

A view can be read only or updatable, that can be determined by its definition.

If a view definition having complete primary key field as well as Not Null key fields then view is updatable otherwise read only.

Example-

Let us take example of each view one by one by following given steps-

1. Create a table student by given command for which you want to create a VIEW.

SQL> CREATE TABLE Student

(

Enroll_No int PRIMARY KEY,

Student_Name varchar(20),

Course varchar(10),

Fees numeric(10,2)

);

2. Create a Read Only VIEW named VWR_Student by given command-

SQL>CREATE OR REPLACE VIEW VWR_Student AS Select Student_Name, Course, Fees FROM Student;

3. Create Updatable VIEW named VWU_Student by given command-

SQL>CREATE OR REPLACE VIEW VWU_Student AS Select * FROM Student;

4. Now we can check the difference between these two views one by one by given commands-

A. We can not perform insert operation by Read Only View because it does not have primary key. If we perform below insert command without primary key data than there will be an error.

SQL>INSERT INTO VWR_Student VALUES('ABC', 'BCA', 12000);

error: can not insert NULL into System.Student.Enroll_No.

B. We can perform below insert command by Updatable View because it has primary key.

SQL>INSERT INTO VWU_Student VALUES(1, 'ABC', 'BCA', 12000);

OUTPUT WINDOW of VIEW

Practical Demonstration of VIEW in Oracle

SEQUENCE

Sequence is an order set of values like 1, 3, 5, … that are generated by SQL Program in Oracle and supported by some database systems to produce unique values on requirement.

A sequence is nothing but a user-defined schema or object that automatic generates an ordered value of numeric type.

Sequences are regularly used in database programming skills now a days because of the requirement of a unique value in each row inside a table in many applications and sequences provides an easy way to generate it.

The order of numeric values are automatically generated in an ascending or descending order according to definition of Sequence with defined intervals and can be restarted when exceeds maximum value.

Example-

Let us understand Sequence in Oracle with following Steps.

1. Execute below command to create a table student for which you want to create a SEQUENCE.

SQL> CREATE TABLE Student

(

Enroll_No int PRIMARY KEY,

Student_Name varchar(20),

Course varchar(10),

Fees numeric(10,2)

);

2. Create a SEQUENCE named SEQ_Student by given command-

SQL>CREATE SEQUENCE SEQ_Student

start with 1

increment by 1

minvalue 0

maxvalue 100

cycle;

3. Insert the sequential value by nextval predefined function given below-

SQL>INSERT INTO Student VALUES(SEQ_Student.nextval, 'ABC', 'BCA', 12000);

SQL>INSERT INTO Student VALUES(SEQ_Student.nextval, 'ABCD', 'BCOM', 14000);

OUTPUT WINDOW of Sequence

Practical Demonstration of Sequence in Oracle

SYNONYMS

The CREATE SYNONYM is a statement which allows you to create an alternative name called synonym for a database object such as a table, sequence, view, procedure and stored function.

Example-

Let us take an Example to understand Synonyms by following steps-

1. Execute below command to create a student table.

SQL> CREATE TABLE Student

(

Enroll_No int PRIMARY KEY,

Student_Name varchar(20),

Course varchar(10),

Fees numeric(10,2)

);

2. Execute below command to insert a record in student table.

SQL>INSERT INTO Student VALUES(1,'abc', 'BCA', 10000);

3. Execute below command to create an alternate name or synonym of student table (ex. Stud).

SQL> CREATE OR REPLACE SYNONYM Stud FOR Student;

4. Execute any one of the below commands to select record from student table.

SQL>SELECT * FROM STUDENT;

SQL>SELECT * FROM STUD;

OUTPUT WINDOW of SYNONYM

Practical Demonstration of Synonym in Oracle

PRATICAL NUMBER-7

➤ Perform Different types of joins on any two tables.

Types of joins-

A. Equi Join

B. Theta Join or Non-Equi Join

C. Self Join

D. Natural Join

E. Inner join

F. Outer Join

1. Left Outer Join

2. Right Outer Join

3. Full Outer Join

G. Cross Join

We can perform these joins by given below steps-

1. Create Student and Course table to perform join operations by given below commands-

SQL>CREATE TABLE Student

(

Enroll_No int,

Student_Name varchar(20),

Course_ID int

);

Table created.

SQL>CREATE TABLE Course

(

Course_ID int,

Course_Name varchar(20),

Course_Duration varchar(20)

);

Table created.

2. Insert records in above table by given below commands-

SQL> INSERT INTO Student VALUES(101, 'Shreya', 102);

1 row created.

SQL> INSERT INTO Student VALUES(102, 'Tanu', 103);

1 row created.

SQL> INSERT INTO Student VALUES(103, 'Roshni', 100);

1 row created.

SQL> INSERT INTO Student VALUES(104, 'Manisha', 101);

1 row created.

SQL> INSERT INTO Course VALUES(101, 'BCA', 'Three Years');

1 row created.

SQL> INSERT INTO Course VALUES(103, 'BEd', 'Two Years');

1 row created.

SQL> INSERT INTO Course VALUES(108, 'BALLB', 'Five Years');

1 row created.

3. Now check the record by below select Command-

SQL>SELECT * FROM Student;

Enroll_No	Student_Name	Course_ID
101	Shreya	102
102	Tanu	103
103	Roshni	100
104	Manisha	101

SQL> SELECT * FROM Course;

Course_ID	Course_Name	Course_Duration
101	BCA	Three Years
103	BEd	Two Years
108	BALLB	Five Years

4. Now we can start to perform the join operations by below commands-

A. Equi Join-It is used to join two tables by equal to operator(=).

SQL> SELECT Enroll_No, Student_Name, Course_Name, Course_Duration FROM Student S, Course C WHERE S.Course_ID = C.Course_ID;

Enroll_No	Student_Name	Course_Name	Course_Duration
102	Tanu	BEd	Two Years
104	Manisha	BCA	Three Years

B. Non-Equi Join-It is used to join two tables by Relational Operator except equal to operator.

SQL> SELECT Enroll_No, Student_Name, Course_Name, Course_Duration FROM Student S, Course C WHERE S.Course_ID > C.Course_ID;

Enroll_No	Student_Name	Course_Name	Course_Duration
101	Shreya	BCA	Two Years
102	Tanu	BCA	Two Years

C. Self Join- It is used to join table itself by its aliases.

SQL> SELECT S1. Enroll_No, S2. Course_ID, S1.Student_Name FROM Student S1, Student S2 WHERE S1.Enroll_No=S2.Course_ID;

Enroll_No	Course_ID	Student_Name
102	102	Tanu
103	103	Roshni
101	101	Shreya

D. Natural Join- It is used to join tables by common Attribute.

SQL> SELECT * FROM Student NATURAL JOIN Course;

Course_ID	Enroll_No	Student_Name	Course_Name	Course_Duration
103	102	Tanu	BEd	Two Years
101	104	Manisha	BCA	Three Years

E. Inner Join- It is used to join tables by common values of conditional result.

SQL> SELECT * FROM Student INNER JOIN Course ON Student.Course_ID=Course.Course_ID;

Enroll_No	Student_Name	Course_ID	Course_ID	Course_Name	Course_Duration
102	Tanu	103	103	BEd	Two Years
104	Manisha	101	101	BCA	Three Years

F. Outer Join- It is used to join tables by common Attribute as well as table remaining attributes based on different outer joins. Let us Discuss all three Outer Joins by an example of each.

1. LEFT OUTER JOIN- It is used to fetch the common result of both tables as well as complete record of Left Table and null values from Right table.

SQL> SELECT * FROM Student LEFT OUTER JOIN Course ON Student.Course_ID = Course.Course_ID;

Enroll_No	Student_Name	Course_ID	Course_ID	Course_Name	Course_Duration
104	Manisha	101	101	BCA	Three Years
102	Tanu	103	103	BEd	Two Years
103	Roshni	100
101	Shreya	102

2. RIGHT OUTER JOIN- It is used to fetch the common result of both tables as well as complete record of Right Table and null values from left table.

SQL> SELECT * FROM Student RIGHT OUTER JOIN Course ON Student.Course_ID = Course.Course_ID;

Enroll_No	Student_Name	Course_ID	Course_ID	Course_Name	Course_Duration
102	Tanu	103	103	BEd	Two Years
104	Manisha	101	101	BCA	Three Years
			108	BALLB	Five Years

3. FULL OUTER JOIN- It is used to fetch the common result of both tables as well as complete record of Left Table and right table with null values from both tables.

SQL> SELECT * FROM Student FULL OUTER JOIN Course ON Student.Course_ID = Course.Course_ID;

Enroll_No	Student_Name	Course_ID	Course_ID	Course_Name	Course_Duration
104	Manisha	101	101	BCA	Three Years
102	Tanu	103	103	BEd	Two Years
103	Roshni	100
101	Shreya	102
			108	BALLB	Five Years

G. Cross Join- It is used to join tables by mapping of each row of first table to each row of second table.

SQL> SELECT * FROM Student CROSS JOIN Course;

Enroll_No	Student_Name	Course_ID	Course_ID	Course_Name	Course_Duration
101	Shreya	102	101	BCA	Three Years
102	Tanu	103	101	BCA	Three Years
103	Roshni	100	101	BCA	Three Years
104	Manisha	101	101	BCA	Three Years
101	Shreya	102	103	BEd	Two Years
102	Tanu	103	103	BEd	Two Years
103	Roshni	100	103	BEd	Two Years
104	Manisha	101	103	BEd	Two Years
101	Shreya	102	108	BALLB	Five Years
102	Tanu	103	108	BALLB	Five Years
103	Roshni	100	108	BALLB	Five Years
104	Manisha	101	108	BALLB	Five Years