Relational Algebra

Question 1

Relational Query Languages do what?

Answer 1

allow the manipulation and retrieval of data from a database

Question 2

two types of query languages

Answer 2

declarative, procedural

Question 3

Declarative query language

Answer 3

describe what a user wants, rather than how to compute it (TRC, DRC)

Question 4

Procedural query language

Answer 4

operational, useful for representing execution plans (relational alg)

Question 5

relational query languages

Answer 5

can’t capture any type of computation, not intended for complex computations

Question 6

algebra

Answer 6

operands - varialbes or values from which new values can be constructed
operators - symbols denoting procedures that construct new values from given values

Question 7

relational algebra

Answer 7

an algebra whose operands are relations or variables that represent relations

Question 8

Selection

Oc(R)

Answer 8

outputs the rows of R that satisfy C

Question 9

Projection

PIa(R)

Answer 9

outputs only the columns A1, A2, …An, removes duplicate tuples

Question 10

Union R1 U R2

Answer 10

outputs all tuples in R1 or R2

Question 11

Difference

R1 - R2

Answer 11

outputs all tuples in R1 and not in R2

Question 12

Cross Product

R1 x R2

Answer 12

matches each tuples in R1 with each tuple in R2

Question 13

Renaming

Pa(R)

Answer 13

does not change the instance, only the schema

Question 14

Intersection

R1 ^ R2

Answer 14

outputs all tuples in R1 and R2

Question 15

Theta Join

Answer 15

Cross product followed by a selection

condition = any boolean valued condition

Question 16

Equi-Join

Answer 16

Condition C contains only equalities between attributes

Question 17

Natural Join

Answer 17

• equi-join on all the common fields

- the output schema has one copy of each common attribute

Question 18

Semi-Join

R1 (bowtie) R2

Answer 18

natural join followed by projection on the attributes of R1

Question 19

Division

R1/R2

Answer 19

all values a such that for every tuple b in R, tuple (a, b) is in R1

Question 20

Group by Aggregate

Answer 20

group by the attributes in X

aggregate the attribute in Y

Question 21

RA cannot express what?

Answer 21

Transitive closure - all pairs of nodes connected by a direct path