Which of the following is true ? I. Implementation of self-join is possible in…
2014
Which of the following is true ?
I. Implementation of self-join is possible in SQL with table alias.
II. Outer-join operation is basic operation in relational algebra.
III. Natural join and outer join operations are equivalent.
- A.
I and II are correct.
- B.
II and III are correct.
- C.
Only III is correct.
- D.
Only I is correct.
Attempted by 253 students.
Show answer & explanation
Correct answer: D
Correct answer: Only statement I is correct.
Statement I: True — a self-join is possible in SQL by giving the same table two different aliases and joining them (for example, to relate employees to their managers stored in the same employee table).
Statement II: False — outer join is not part of the original set of basic operations in classical relational algebra; it is an extension. The classic basic operations include selection, projection, union, set difference, Cartesian product, and rename (joins can be expressed using these).
Statement III: False — natural join and outer join are different. A natural join returns only tuples with matching values on common attributes. An outer join (left, right, or full) includes non-matching tuples as well, filling missing attributes with NULLs. Therefore they are not equivalent.
Summary: Only the first statement is correct; the second and third statements are incorrect for the reasons given above.