GATE CSE 2007 | Question: 59

Question

Information about a collection of students is given by the relation $\text{studInfo(}\underline{\text{studId}},\text{ name, sex)}$. The relation $\text{enroll(}{\text{studId}},{\text{ courseId}})$ gives which student has enrolled for (or taken) what course(s). Assume that every course is taken by at least one male and at least one female student. What does the following relational algebra expression represent?

$\pi _{courceId}\left(\left(\pi_{\text{studId}}\left(\sigma_{sex=“female"}\left(\text{studInfo}\right)\right) \times \pi_{courseId}\left(\text{enroll}\right)\right) -\text{enroll}\right)$

• A. Courses in which all the female students are enrolled.
• B. Courses in which a proper subset of female students are enrolled.
• C. Courses in which only male students are enrolled.
• D. None of the above

Comments

You can follow the naming GATEYYYY_NN for the question title (YYYY- year and NN question number) and it will show if the question is already there. Now all CS GATE papers from 1997-2014 are there. This question was already there, but I removed that.

Answer 1

As simple as possible, Thanks :)

Answer 2

proper subset means subset not equal to set itself.......so above relational algebra is going to give all courses that "all girls have not taken"

Answer 3

(pie_sex=female(studid))cross pie_courseid(enroll) it gives relation with four attribute(studid,name,sex,enrollid) but it contains only information about female

and if we subtract enroll relTION FROM IT THEN IN SUBSTRACTION ONLY THOSE TUPLES WILL EXTRACTED WHICH ARE NOT IN (ENROLL) but enroll have all because it is original relation so it gives nothing so (d) will be answer 

Comments

As explained by @Anurag_s above answer should be b)

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u r right

sorry i hav written wrong in place of sigma i have written pie sorry to disturb u alll