* A. A customer can exist in many countries. This is true as customers can have multiple addresses or operations in different countries, and a database design can reflect this by allowing multiple country entries for a single customer1.
* C. The statement will fail because subquery may not be I contained in a values clause. In Oracle Database 12c, a subquery cannot be used within the VALUES clause of an INSERT statement. The correct approach would be to use the subquery in conjunction with the INSERT INTO ... SELECT syntax if multiple rows are derived from a subquery2.
* F. A product can have a different unit price at different times. It is common for products to have different unit prices at different times due to various factors such as promotions, discounts, or changes in cost price. This can be represented in a database by having a price history table or a similar mechanism to track the changes in price over time1.
Note: The other options are incorrect because:
* B. The statement about the SALES table failing if a row already exists for product 23 is not necessarily true. Oracle allows for multiple rows with the same product ID if the table is designed to handle such cases, like having a composite primary key or no constraints preventing duplicates.
* D. Without specific information about the SALES table's design, we cannot verify the number of foreign keys it has.
* E. The statement about the successful execution and insertion of a new row into the SALES table is too vague without the context of the actual SQL statement being referred to.

E). True. The deptno column in the emp table can contain null values unless explicitly constrained not to, which is not the case here.
F). True. The three statements can execute successfully in the order shown. The foreign key constraint in the dept table is referring to the emp table which does not yet exist, but this is not a problem because it is not immediately enabled. The emp table is created next with a disabled foreign key constraint referencing the already created dept table. Finally, the foreign key constraint on emp is enabled after both tables are in place.
A is incorrect because the MGR column can contain NULL values as there is no NOT NULL constraint applied to it. B is incorrect because the CREATE TABLE EMP statement contains a disabled foreign key, so it can be created before CREATE TABLE DEPT. C and D are incorrect because foreign key constraints do not need to be removed and added later for the tables to be created successfully; they can be managed as shown in the provided statements.

C: True. A UNIQUE constraint in Oracle SQL allows for the inclusion of NULL values; specifically, it permits multiple NULLs in a column or set of columns but ensures that all non-NULL values are unique.
D: True. A PRIMARY KEY constraint can indeed be added to a table after it has been created and even after it has been populated, as long as the existing data does not violate the primary key constraint rules (i.e., all values must be unique and not NULL).
F: True. A UNIQUE constraint can utilize a pre-existing index on the columns it covers. If a suitable index already exists, Oracle can use this index to enforce the constraint, optimizing performance and resource utilization.
G: True. Columns that are part of a FOREIGN KEY constraint can contain NULL values. This is permissible under SQL standards and Oracle implementation, as a NULL foreign key value is considered to not refer to any row in the referenced table and thus does not violate referential integrity.

To calculate the length of employment, you would compare the current date to the HIRE_DATE and calculate the difference in years.
A . This is the correct answer. The expression calculates the number of days between the current date (SYSDATE) and the HIRE_DATE, then divides by 365 to convert it to years, and checks if it is greater than 5.
B . SYSTIMESTAMP includes a time component including fractional seconds and time zone information, making this comparison incorrect.
C . There are typos in this option (CUARENT_DATE should be CURRENT_DATE and hire_data should be hire_date). Even with the correct function and column names, CURRENT_DATE returns the current date in the session time zone, not in years.
D . There are typos in this option (SYSNAYW should be SYSDATE). Additionally, dividing by 12 incorrectly assumes that a month is equivalent to a year.
E . This is a repetition of option D with the same issues.
F . There are typos in this option (CUNACV_DATE should be CURRENT_DATE and hire_data should be hire_date). Additionally, dividing by 12 incorrectly assumes that a month is equivalent to a year.
Reference:
Oracle Database SQL Language Reference, 12c Release 1 (12.1): "Date Functions"