To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) The SQL statement displays the desired results.
This option is incorrect because the SQL statement will not display the desired results.
Option B) The column in the WHERE clause should be changed to display the desired results.
This option is incorrect because the column in the WHERE clause is correct. The column 'DEPARTMENT_ID' is being checked for NULL values, which is the correct approach.
Option C) The operator in the WHERE clause should be changed to display the desired results.
This option is correct because the operator used in the WHERE clause is incorrect. To check for NULL values, you should use the 'IS NULL' operator instead of the '=' operator. So the correct SQL statement should be:
SELECT LAST_NAME, SALARY, DEPARTMENT_ID FROM EMP WHERE DEPARTMENT_ID IS NULL;
Option D) The WHERE clause should be changed to use an outer join to display the desired results.
This option is incorrect because using an outer join is not necessary to display the desired results. The problem lies in the operator used in the WHERE clause, not the join type.
The correct answer is Option C. The operator in the WHERE clause should be changed to use 'IS NULL' instead of '=' to display the desired results.