Question

Consider a database that has the relation schema CR(StudentName. CourseName). An instance of the schema CR is as given below.

CR
Student Name	Course Name
SA	CA
SA	CB
SA	CC
SB	CB
SB	CC
SC	CA
SC	CB
SC	CC
SD	CA
SD	CB
SD	CC
SD	CD
SE	CD
SE	CA
SE	CB
SF	CA
SF	CB
SF	CC

 

The following query is made on the database.

T1 ← π_CourseNane(σ_{StudentName=’SA’}(CR))

T2 ← CR ÷ T1
The number of rows in T2 is _______. 

Answer 1

Explanation: Query T1

T1 ← π_CourseNane(σ_{StudentName=’SA’}(CR))

Output of this query are the courses enrolled by student SA

Output of T1

CA

CB

CC

 

Query T2: T2 ← CR ÷ T1
It results in the name of the students who enrol in all the courses that are the result of query T1 .

Output of query T2

SA

SC

SD

SF

 

So, the number of rows returned by T2 are 4.