Question

Which clause is used to combine rows from two tables that meet a condition?

• A. UNION
• B. INTERSECT
• C. JOIN
• D. MINUS

Hint:

Which keyword takes an ON condition to pair the rows of two tables?

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Relational databases keep related data in separate tables. To read related data together, we combine rows of two tables on a matching condition, for example emp.dept_id = dept.id.

Step 2: Key Rule:
The clause that combines rows of two tables using a condition is JOIN. Set operators such as UNION, INTERSECT and MINUS work on whole result sets and use no join condition.

Step 3: Check option 1 (UNION).
UNION stacks the rows of two queries and removes duplicates. It has no condition that matches rows of one table with rows of another. So option 1 is wrong.

Step 4: Check option 2 (INTERSECT).
INTERSECT returns only the rows that appear in both query results. It compares whole rows and does not pair rows by a condition. So option 2 is wrong.

Step 5: Check option 3 (JOIN).
JOIN pairs each row of one table with the rows of the other table that satisfy the given condition, written with ON or WHERE. This is the described clause. So option 3 is correct.

Step 6: Check option 4 (MINUS).
MINUS returns the rows of the first query that are not in the second. It removes rows and does not combine them on a condition. So option 4 is wrong.

Final Answer:
Combining rows of two tables on a condition is done by JOIN, which is option 3. \[ \boxed{3} \]