GATE CS 2017 Set 1 — Question 46
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Databases → Relational Model → Division
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Question
Consider a database that has the relation schema CR(StudentName, CourseName). An instance of the schema CR is as given below.
The following query is made on the database.The number of rows in is ________.
| StudentName | CourseName |
|---|---|
| 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 |
Correct answer
4 to 4
Solution
1.Evaluate the subquery for :
From the given instance of the relation CR, the student 'SA' is enrolled in the courses {CA, CB, CC}. Therefore, the set contains these three courses:
2.Evaluate the division operation for :
The relational division operator returns all values of from that are associated with every value in . In this context, will contain the names of students who are enrolled in all courses listed in (i.e., CA, CB, and CC).
3.Check each student's enrollment in CR:
- SA: Enrolled in {CA, CB, CC}. (Matches all in )
- SB: Enrolled in {CB, CC}. (Missing CA)
- SC: Enrolled in {CA, CB, CC}. (Matches all in )
- SD: Enrolled in {CA, CB, CC, CD}. (Matches all in )
- SE: Enrolled in {CD, CA, CB}. (Missing CC)
- SF: Enrolled in {CA, CB, CC}. (Matches all in )
The students who satisfy the condition are {SA, SC, SD, SF}. Thus, there are 4 rows in the resulting relation .
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