When using the GROUP BY clause, every column in the SELECT clause that is not an aggregate function must be included in the GROUP BY clause:
* A. SELECT dept_id, join_date, SUM(salary) FROM employees GROUP BY dept_id, join_date:
This is a valid query because all non-aggregate columns in the SELECT list (dept_id and join_date) are included in the GROUP BY clause.
Queries B and C are invalid because they attempt to nest aggregate functions, which is not allowed. Query D is invalid because join_date is not included in the GROUP BY clause.
References:
* Oracle Database SQL Language Reference 12c, specifically the section on GROUP BY clause constraints.

Explanation/Reference:
References:
http://www.oracle.com/technetwork/issue-archive/2011/11-nov/o61sql-512018.html

A: True. TRUNCATE is generally faster than DELETE for removing all rows from a table because TRUNCATE is a DDL (Data Definition Language) operation that minimally logs the action and does not generate rollback information. TRUNCATE drops and re-creates the table, which is much quicker than deleting rows one by one as DELETE does, especially for large tables. Also, TRUNCATE does not fire triggers.
Reference:
Oracle documentation specifies that TRUNCATE is faster because it doesn't generate redo logs for each row as DELETE would.
TRUNCATE cannot be rolled back once executed, since it is a DDL command and does not generate rollback information as DML commands do.