GATE | GATE-CS-2006 | Question 68
Consider the relation “enrolled(student, course)” in which (student, course) is the primary key, and the relation “paid(student, amount)” where student is the primary key. Assume no null values and no foreign keys or integrity constraints. Given the following four queries:
Query1: select student from enrolled where
student in (select student from paid)
Query2: select student from paid where
student in (select student from enrolled)
Query3: select E.student from enrolled E, paid P
where E.student = P.student
Query4: select student from paid where exists
(select * from enrolled where enrolled.student
= paid.student)
Which one of the following statements is correct?
(A) All queries return identical row sets for any database
(B) Query2 and Query4 return identical row sets for all databases but there exist databases for which Query1 and Query2 return different row sets.
(C) There exist databases for which Query3 returns strictly fewer rows than Query2
(D) There exist databases for which Query4 will encounter an integrity violation at runtime.
Answer: (B)
Explanation: Take an example:
Table enrolled
student course
----------------
abc c1
xyz c1
abc c2
pqr c1
Table paid
student amount
-----------------
abc 20000
xyz 10000
rst 10000
Output of Query 1
abc
abc
xyz
Output of Query 2
abc
xyz
Output of Query 3
abc
xyz
Output of Query 4
abc
xyz
Query 1 and Query 3 may return repetitive student values as “student” is not a key in relation enrolled, however query 2 and query 4 always return same row sets.
So, option (B) is correct.
Quiz of this Question
Last Updated :
12 Apr, 2018
Like Article
Save Article
Share your thoughts in the comments
Please Login to comment...