C: True. A SQL statement may include multiple single row subqueries in different parts of the statement, such as in the SELECT list, WHERE clause, or HAVING clause. Each subquery must independently satisfy the requirement of returning a single row to avoid runtime errors.
D: True. Single row subqueries can be used in the HAVING clause. This allows for filtering groups based on conditions evaluated against individual or aggregated values returned by the subquery. The subquery must return a single value to be valid in this context.
E: True. Single row subqueries are often placed on the right side of the comparison operator in a SQL condition. This positioning is typical because the left side often references a column or an expression related to the main query, while the subquery on the right side dynamically provides a value for comparison.

Regarding Oracle join and ANSI join syntax:
B . The Oracle join syntax supports the creation of a Cartesian product of two tables. This is true. In Oracle, if you list tables in the FROM clause without a join condition, it creates a Cartesian product.
C . The SQL:1999 compliant ANSI join syntax supports natural joins. This is true. ANSI syntax supports natural joins, which join tables based on columns with the same names in the joined tables.
F . The SQL:1999 compliant ANSI join syntax supports the creation of a Cartesian product of two tables. This is true. The ANSI standard allows for Cartesian products when tables are listed in the FROM clause without a join condition.
Options A, D, E, and G are incorrect:
A is incorrect because the Oracle join syntax supports all types of joins, including right outer joins.
D is incorrect because Oracle's proprietary join syntax does not use the term "natural join." E is incorrect because there is no inherent performance difference between Oracle join syntax and ANSI join syntax; performance depends on how the query is written and how the database optimizer handles it.
G is incorrect for the same reason as E.

D). True, after setting the DEPARTMENT_ID column to UNUSED, you can add a new column with the name DEPARTMENT_ID to the EMPLOYEES table. The UNUSED clause does not delete the column, it only marks it as no longer being used.E. True, once a column is marked as UNUSED, you cannot make updates to it. It becomes inaccessible for DML operations.
A, B, C, and F are not correct because: A. Once a column is set to UNUSED, it is not available for queries. B.
The storage space is not immediately released after issuing a COMMIT; instead, the actual removal and space reclamation happen when you subsequently issue the DROP UNUSED COLUMNS operation. C. The DEPARTMENT_ID column is not set to null; instead, it's marked as UNUSED, which means it is no longer available for use. F. UNUSED columns are not placed into the recycle bin; they are just marked for deletion, and space can be reclaimed with the DROP UNUSED COLUMNS command.
References:
* Oracle documentation on ALTER TABLE: Oracle Database SQL Language Reference
* Understanding ALTER TABLE SET UNUSED: Oracle Database Administrator's Guide