Relational Algebra

Relational algebra is a procedural query language. It gives a step by step process to obtain the result of the query. It uses operators to perform queries.

Types of Relational operation

1. Select Operation:

• The select operation selects tuples that satisfy a given predicate.
• It is denoted by sigma (σ).

1.                               Notation:  σ p(r)  

Where:

σ is used for selection prediction
r is used for relation
p is used as a propositional logic formula which may use connectors like: AND OR and NOT. These relational can use as relational operators like =, ≠, ≥, <, >, ≤.

For example: LOAN Relation

BRANCH_NAME	LOAN_NO	AMOUNT
Downtown	L-17	1000
Redwood	L-23	2000
Perryride	L-15	1500
Downtown	L-14	1500
Mianus	L-13	500
Roundhill	L-11	900
Perryride	L-16	1300

Input:

1.      σ BRANCH_NAME="perryride" (LOAN)  

Output:

BRANCH_NAME	LOAN_NO	AMOUNT
Perryride	L-15	1500
Perryride	L-16	1300

2. Project Operation:

• This operation shows the list of those attributes that we wish to appear in the result. Rest of the attributes are eliminated from the table.
• It is denoted by ∏.

1.      Notation: ∏ A1, A2, An (r)     

Where

A1, A2, A3 is used as an attribute name of relation r.

Example: CUSTOMER RELATION

NAME	STREET	CITY
Jones	Main	Harrison
Smith	North	Rye
Hays	Main	Harrison
Curry	North	Rye
Johnson	Alma	Brooklyn
Brooks	Senator	Brooklyn

Input:

1.      ∏ NAME, CITY (CUSTOMER)  

Output:

3. Union Operation:

• Suppose there are two tuples R and S. The union operation contains all the tuples that are either in R or S or both in R & S.
• It eliminates the duplicate tuples. It is denoted by ∪.

1.      Notation: R ∪ S   

A union operation must hold the following condition:

• R and S must have the attribute of the same number.
• Duplicate tuples are eliminated automatically.

Example:

DEPOSITOR RELATION

CUSTOMER_NAME	ACCOUNT_NO
Johnson	A-101
Smith	A-121
Mayes	A-321
Turner	A-176
Johnson	A-273
Jones	A-472
Lindsay	A-284

BORROW RELATION

CUSTOMER_NAME	LOAN_NO
Jones	L-17
Smith	L-23
Hayes	L-15
Jackson	L-14
Curry	L-93
Smith	L-11
Williams	L-17

Input:

1.      ∏ CUSTOMER_NAME (BORROW) ∪ ∏ CUSTOMER_NAME (DEPOSITOR)  

Output:

CUSTOMER_NAME

Johnson

Smith

Hayes

Turner

Jones

Lindsay

Jackson

Curry

Williams

Mayes

4. Set Intersection:

• Suppose there are two tuples R and S. The set intersection operation contains all tuples that are in both R & S.
• It is denoted by intersection ∩.

1.      Notation: R ∩ S   

Example: Using the above DEPOSITOR table and BORROW table

Input:

1.      ∏ CUSTOMER_NAME (BORROW) ∩ ∏ CUSTOMER_NAME (DEPOSITOR)  

Output:

CUSTOMER_NAME

Smith

Jones

5. Set Difference:

• Suppose there are two tuples R and S. The set intersection operation contains all tuples that are in R but not in S.
• It is denoted by intersection minus (-).

1.      Notation: R - S  

Example: Using the above DEPOSITOR table and BORROW table

Input:

1.      ∏ CUSTOMER_NAME (BORROW) - ∏ CUSTOMER_NAME (DEPOSITOR)  

Output:

CUSTOMER_NAME

Jackson

Hayes

Willians

Curry

6. Cartesian product

• The Cartesian product is used to combine each row in one table with each row in the other table. It is also known as a cross product.
• It is denoted by X.

1.      Notation: E X D  

Example:

EMPLOYEE

EMP_ID	EMP_NAME	EMP_DEPT
1	Smith	A
2	Harry	C
3	John	B

DEPARTMENT

DEPT_NO	DEPT_NAME
A	Marketing
B	Sales
C	Legal

Input:

1.      EMPLOYEE X DEPARTMENT  

Output:

EMP_ID	EMP_NAME	EMP_DEPT	DEPT_NO	DEPT_NAME
1	Smith	A	A	Marketing
1	Smith	A	B	Sales
1	Smith	A	C	Legal
2	Harry	C	A	Marketing
2	Harry	C	B	Sales
2	Harry	C	C	Legal
3	John	B	A	Marketing
3	John	B	B	Sales
3	John	B	C	Legal

7. Rename Operation:

The rename operation is used to rename the output relation. It is denoted by rho (ρ).

Example: We can use the rename operator to rename STUDENT relation to STUDENT1.

1.      ρ(STUDENT1, STUDENT)  

Note: Apart from these common operations Relational algebra can be used in Join operations.