ITPEC FE Morning October 2018 Question 29

Topic: Databases

After relations X and Y are joined, which of the following is (are) the relational algebra operation(s) to obtain relation Z?

X

StudentNumber	Name	FacultyCode
1	Amy White	A
2	Bob Green	B
3	Cathy Black	A
4	David Grey	B
5	Edward Brown	A
6	Frank Blue	A

Y

FacultyCode	FacultyName
A	Engineering
B	Information
C	Literature

Z

FacultyName	StudentNumber	Name
Information	2	Bob Green
Information	4	David Grey

a)Projection and selectionb)Projection and unionc)Selectiond)Selection and union

Explanation

Projection and selection — after the join, selection keeps only the Information rows and projection keeps only three columns.

Joining X and Y on FacultyCode gives six rows with the columns StudentNumber, Name, FacultyCode and FacultyName (faculty C has no students, so it drops out). Relation Z has two rows and three columns:

•Selection picks the rows where FacultyName is Information: students 2 and 4
•Projection keeps the columns FacultyName, StudentNumber and Name, dropping FacultyCode

Answer (a)

Why not others:

•(b) Projection and union — union adds the rows of a second relation with the same columns; it can never reduce six rows to two
•(c) Selection — reduces the rows but leaves FacultyCode in the result, which Z does not have
•(d) Selection and union — still no projection to remove FacultyCode, and there is no second relation to unite with

Key rule: Selection chooses rows (the WHERE clause), projection chooses columns (the SELECT list), and union combines rows of two compatible relations.

Databases · AI-written, may contain errors

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