Final answer:
The correct answer to the question is (d) q and s. The HAVING clause requires a GROUP BY clause to have been specified, but not all attributes used in the GROUP BY clause are required to be in the SELECT clause.
Step-by-step explanation:
The question is regarding SQL queries and the usage of HAVING and GROUP BY clauses, as well as the relationship between the SELECT clause and the GROUP BY clause.
To answer the student's question:
- q: An SQL query can contain a HAVING clause only if it has a GROUP BY clause. This statement is factually correct. The HAVING clause is used to filter groups created by the GROUP BY clause and cannot be used without it.
- r: All attributes used in the GROUP BY clause must appear in the SELECT clause. This statement is not correct. The SELECT clause can contain additional expressions or aggregate functions that are not part of the GROUP BY clause.
- s: Not all attributes used in the GROUP BY clause need to appear in the SELECT clause. This is correct. You can include other expressions in the SELECT clause that are not part of the GROUP BY clause, often involving aggregate functions like COUNT, SUM, AVG, etc.
Therefore, the correct answer to the student's question is (d) q and s.