Lecture 5- Relational Algebra

Question 1

What are the three variants of relational query language?

Answer 1

1. Relational algebra
2. Relational calculus
3. SQL

Question 2

What is algebra?

Answer 2

The study of operations in an abstract level on some domains (as a language, study of syntax and semantics of expressions)

Question 3

What are two parts of relational algebra?

Answer 3

1. Domain= set of all relations
2. Expressions= referred to as queries

Question 4

Operations on one table vs two tables?

Answer 4

• Operations on one table= pick some rows, pick some columns
• Operations on two tables= join

Question 5

What is the domain, basic operations and derived operators?

Answer 5

1. Domain = set of relations
2. Basic operations = select, project, union, set difference, cartesian product, renaming
3. Derived operators = set intersection, division, join

Question 6

Describe the role of algebra inside of a DBMS

Answer 6

Question 7

What is a select operator?

Answer 7

Select rows that satisfy the condition

Question 8

This is an example of what?

Answer 8

Select operator

Question 9

This is an example of what?

Answer 9

Select operator

Question 10

What are the operators for select condition?

Answer 10

Question 11

What are some simple selection conditions?

Answer 11

• < attribute> operator < constant>
• < attribute> operator < attribute>
• < condition> AND < condition>
• < condition> OR < condition>
• NOT < condition>

Question 12

What is a project operator?

Answer 12

Project on (or pick) a subset of columns

Question 13

This is an example of what?

Answer 13

Project operator

Question 14

This is an example of what?

Answer 14

Project operator

Question 15

What are some set operator operations?

Answer 15

Question 16

Two relations are union compatible if?

Answer 16

• Both have the same number of columns
• Names of attributes with the same name in both relations have the same domain

Question 17

Union compatible relations can be combined using?

Answer 17

• Union
• Intersection
• Set difference

Question 18

This is an example of what?

Answer 18

Set operator

Question 19

What is the cartesian product?

Answer 19

R x S is the set of all concatenated tuples < x,y>, where x is a tuple in R and y is a tuple in S

Question 20

For cartesian products, relations ________

Answer 20

Don’t have to be union compatible

Question 21

R x S can be ____ and expensive to compute

Answer 21

Huge

Question 22

How to compute the cartesian product?

Answer 22

Question 23

What is renaming?

Answer 23

• Expr returns a relation
• Rename the resulting relations to x with the first column in the result relation to A1, the second to A2, etc

Question 24

This is an example of what?

Answer 24

Renaming

Question 25

This is an example of what?

Answer 25

Renaming

Question 26

What is a common usage of renaming?

Answer 26

* To clean up the result * To prepare the result for the next operation

Question 27

What is join (derived operator)?

Answer 27

A (general or theta) join of R and S is the expression

Question 28

Join definition is equivalent to what?

Answer 28

Question 29

This is an example of what?

Answer 29

Join (derived operator)

Question 30

This is an example of what?

Answer 30

Join (derived operator)

Question 31

What is equijoin?

Answer 31

* A special case of condition join where the condition contains only equalities * Result schema similar to cross product, but only one copy of fields for which equality is specified * Equijoin= join condition is a conjunction of equalities

Question 32

This is an example of what?

Answer 32

Equijoin

Question 33

What is a problem and solution to join?

Answer 33

* Problem= if R and S have attributes with the same then the Cartesian product is not well defined * Solution 1= rename attributes prior to forming the product and use new names in join-condition * Solution 2= common attribute names are qualified with relation names in the result of the join

Question 34

What is natural join?

Answer 34

* Special case of equijoin * Join condition equates all and only those attributes with the same name (condition doesn't have to be explicitly stated) * Duplicate columns eliminated from the result

Question 35

This is an example of what?

Answer 35

Natural join

Question 36

What is division?

Answer 36

Finds tuples in one relation r that match all tuples in another relation s (can be expressed in terms of projection, set difference and cross product)

Question 37

This is an example of what?

Answer 37

Division (derived operator)

Question 38

This is an example of what?

Answer 38

Division (derived operator)