RELATIONAL ALGEBRA

Question 1

Define relational algebra

Answer 1

Relational algebra is a procedural query language which tasks instances of relations as input and yields instances of relations as output

Question 2

Define relational calculus

Answer 2

Relational Calculus is a non-procedural query language ie it tells what to do but never explains how to do it

Question 3

What are the differences between relational algebra and relational calculus?

Answer 3

1. Relaional algebra is a procedural language while RC is a declarative language
2. RA states how to obtain the result while RC states what result we have to obtain
3. RA describes the order in which operations have to be performed while RC does not specify the order of operations
4. RA is not domain dependent (data types allowed- domain) while relational Calculus ca be domain dependent
5. RA is close to a programming language while RC is closer to natural language

Question 4

Explain the projection operation And give its syntax

Answer 4

Returns columns from relation.
π (name-of-column) name-or relation

*Check Liban’s notes for syntax

Question 5

Explain the select operation And give its syntax

Answer 5

Selects a subset of rows from relation.

*Check Liban’s notes for syntax

Question 6

Explain the union relation And give its syntax

Answer 6

Tuples in reln. 1 and in reln. 2.

*Check Liban’s notes for syntax

Question 7

Explain set difference And give its syntax

Answer 7

Tuples in reln. 1, but not in reln. 2.

*Check Liban’s notes for syntax

Question 8

Explain Cartesian product And give its syntax

Answer 8

Allows us to combine two relations.

*Check Liban’s notes for syntax

Question 9

What is union compatibility?

Answer 9

Same number of fields.
‘Corresponding’ fields have the same data type.

*This condition is necessary for union, set difference and intersection operations

Question 10

Explain the rename operation And give its syntax

Answer 10

If renaming table don’t put it in brackets if renaming columns place them in brackets Eg
ρ pro(A,B) (Project) -Query to rename the table name Project to Pro and its attributes to A B

*Check Liban’s notes for syntax

Question 11

Describe what a join is and give its different types

Answer 11

Can be defined as cross-product followed by selection
and projection.
• We have several variants of join.
✓ Condition joins
✓ Equijoin
✓ Natural join

Question 12

Describe the theta/ condition join And give its syntax

Answer 12

combines two relations based on specified condition(s)

*see Liban’s notes for syntax

Question 13

Describe the equijoin And give its syntax

Answer 13

A special case of condition join where the condition c
contains only equalities.

*Result schema similar to cross-product, but only one copy of fields for which equality is specified. *Check notes for syntax

Question 14

Describe the natural join And give its syntax

Answer 14

Equijoin on all common fields

*Check Liban’s notes for syntax