Question

Which of the following are to be considered while applying JOIN operations on two or more relations?
A. If two tables are to be joined on equality condition on the common attribute, then one may use JOIN with ON clause or NATURAL JOIN in FROM clause.
B. If three tables are to be joined on equality condition, then two JOIN or NATURAL JOIN are required.
C. N-1 joins are needed to combine N tables on equality condition.
D. With JOIN clause, we may use any relational operators to combine tuples of two tables.
Choose the correct answer from the options given below:

• A. A, B and D only
• B. A, B and C only
• C. A, B, C and D
• D. B, C and D only

Hint:

To link $N$ tables together, you always require at least $N-1$ distinct join conditions.
Fewer than $N-1$ conditions will result in an undesirable Cartesian product!

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
SQL JOIN operations are used to combine records from two or more tables based on a related column between them.

Step 2: Detailed Explanation:

• A is correct: The ‘JOIN...ON‘ clause and ‘NATURAL JOIN‘ are standard methods to join tables based on equality.
• B is correct: To connect three tables (e.g., A, B, and C), you need at least two join operations (e.g., A join B, then the result join C).
• C is correct: For \( N \) tables to be connected, a minimum of \( N-1 \) join links are required to form a connected structure.
• D is correct: A ‘JOIN‘ clause (specifically ‘INNER JOIN‘ or ‘THETA JOIN‘) allows the use of various relational operators (like \( , \le, \ge, \neq \)) in the join condition, not just the equality (‘=‘) operator.

Step 3: Final Answer:
All statements A, B, C, and D are correct.