Relational Algebra

Question 1

Unary Relational Operations

Answer 1

Operations on one set:

Select: sigma: σ
Project: pi: π
Rename: rho: ρ

Question 2

Select: σ

Answer 2

Selects entire entries from a table

Question 3

Project: π

Answer 3

Selects only specified attributes

Question 4

Rename: ρ

Answer 4

Renames attributes or entities

Question 5

Binary Operations:

Answer 5

Operations on two sets:

Union: U
Intersection: ∩
Set Difference: -

Question 6

Union

Answer 6

Includes all tuples from R and S, or R, or S

Question 7

Intersection

Answer 7

Includes all tuples in R and S

Question 8

Set Difference:

Answer 8

Includes all the tuples in R but not S (R-S)

Question 9

Commutative Operatiosn

Answer 9

Union and Intersection

Question 10

Joins:

Answer 10

Binary Operators:

Cross Product: X
Theta: Θ
Equi-join Θ=
Natural Join: *

Question 11

Cartesian Product / Cross Product

Answer 11

Exhausting pairing

Question 12

Theta Join

Answer 12

A join based on any sort of equality, can be combined with ANDs

Question 13

Equijoin

Answer 13

A join based on only strict equality pairing

Question 14

Natural Join

Answer 14

Removes superfluous attributes in an equijoin situation

Question 15

Outer Joins

Answer 15

Keeps tuples in R even if they aren’t in S

Question 16

Left Outer Join

Answer 16

Keep every tuple in the left relation, pad with nulls

Question 17

Right Outer Join

Answer 17

Keep every tuple in the right relation, pad with nulls

Question 18

Full Outer Join

Answer 18

Keeps every tuple in right and left relation regardless of matches, pad with nulls

Question 19

Aggregate Functions

Answer 19

F(Max)
F(Min)
F(Sum)
F(Count)
F(Average)

Question 20

Grouping with Aggregate Functions

Answer 20

Given a relation: EMP(EMPNO, DNO, SAL)

DNO F(AVERAGE)SAL(EMP)

Groups by the DNO

Question 21

Query Trees

Answer 21

A data structure that corresponds to the relation algebra expression