Special relational operations  Relational Model
SPECIAL RELATIONAL OPERATIONS
These includes operations like
REPRESENTATION s (CONDITION) (RELATION)
EXAMPLE Consider a relation "EMP"
Ex1 Select all employees whose salary>=8000
These includes operations like
 Selection
 Projection
 Join
 Division
1. SelectionTo
identify a set of tuples which is a part of a relation and to extract only
these tuples out. The select operation selects tuples that satisfy a given
predicate or condition. It is also known as restriction operation.
It is a unary operation
defined on a single relation.
· It is denoted as s.
REPRESENTATION s (CONDITION) (RELATION)
EXAMPLE Consider a relation "EMP"
ID

NAME

SALARY

1

Rahul

5000

2

Arjun

10000

3

Abay

4000

4

Avinash

8000

5

Anu

10000

Ex1 Select all employees whose salary>=8000
s (salary>8000) (EMP)
OUTPUT
ID

NAME

SALARY

2

Arjun

10000

4

Avinash

8000

5

Anu

10000

2. Projection It results in a new relation that contains a subset of the columns of a relation and eliminate any duplicate node.
·
It is a unary operation
defined on a single relation
It is denoted as ?.
REPRESENTATION ? (COLUMN) (RELATION)
EX2 select all employees name and their salary from the relation "EMP"
? (NAME,SALARY) (EMP)
OUTPUT
NAME

SALARY

Rahul

5000

Arjun

10000

Abay

4000

Avinash

8000

Anu

10000

3. Join To
restrict the no of rows obtained from the Cartesian product we used
joins.Join is performed on 2 relations having one or more attributes in
common and they should have some datatype.It is a
binary operation and a combination of certain selections and a Cartesian
product into one operation.
·
It is denoted as X .
·
It is associative.
REPRESENTATION (P) X (JOIN CONDITION) (Q)
where P,Q are the names of the relation and where condition is of the form pi Q qi
where pi is the attributes of the relation P and qi is the attribute of the relation Q.
EXAMPLE Consider 2 relations
1 relation "EMP"
EMP_ID

ENAME

DEPTID

SAL

123

Balwinder

1

5000

122

Simar

2

10000

134

Anmol

3

20000

2 relation "DEPT"
DEPTNO

DNAME

1

CSE

2

ECE

3

CSE

Ex Display the employee information with department name
EMP  X (DEPID=DEPTNO) DEPT
OUTPUT
EMP_ID

ENAME

DEPTID

SAL

DNAME

123

Balwinder

1

5000

CSE

122

Simar

2

10000

ECE

134

Anmol

3

20000

CSE

4. Division
The division operator divides a dividend relation "r" of degree (m+n) by
a divisor relation "s" of degree (m) and produces a resultant relation
of degree "n".
REPRESENTATION Let r(R) and s(S) be relations
r ÷
s: 
The result
consists of the restrictions of tuples in r to the attribute names unique to R,
i.e. in the Header of r but not in the Header of s, for which it holds that all
their combinations with tuples in s are present in r.
EXAMPLE Consider 2 relations "r" and "s"
Relation"r"
S.NO

P.NO

S1

P1

S1

P2

S3

P1

S4

P2

S2

P3

S2

P1

Relation "s"
P.NO

P1

Now r ÷ s
OUTPUT
S.NO

S1

S3

S2

Good article shared about DBMS..
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