Question

Which of the following operations can be performed with the UNION clause in SQL?

• A. Fetch only common rows between two tables
• B. Combine rows from two tables without duplicates
• C. Combine rows from two tables allowing duplicates
• D. Combine columns with duplicates

Hint:

Think of what plain UNION does with a repeated row. UNION ALL is the one that keeps duplicates.

Answer 1

The Correct Option is B

Step 1: What the question asks.
We must pick the one statement that correctly describes the UNION clause of SQL.

Step 2: Work with a small example.
Let table A hold the rows 1, 2, 3 and table B hold the rows 3, 4. Then SELECT x FROM A UNION SELECT x FROM B gives 1, 2, 3, 4. The row 3 appears only once.

Step 3: Compare with UNION ALL.
With UNION ALL the same example gives 1, 2, 3, 3, 4. The row 3 is repeated. So the duplicate-free result belongs to plain UNION.

Step 4: Compare with INTERSECT.
INTERSECT on the same example gives only 3, the common row. This is the meaning of option 1, so option 1 describes INTERSECT and not UNION.

Step 5: Compare with JOIN.
A join or Cartesian product widens the result by adding columns of both tables. UNION never adds columns, so option 4 is not UNION.

Step 6: Match with the options.
The example shows rows combined and duplicates removed. Only option 2 says this, while option 3 describes UNION ALL.

Final Answer:
The correct statement is that UNION combines rows from two tables without duplicates. \[ \boxed{2} \]