Database-Management-System

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?