Final answer:
An outer join is useful for showing all employees in a company, regardless of their department association. It allows the inclusion of records without corresponding entries in the related table, which isn't possible with an inner join or anti-join.
Step-by-step explanation:
An outer join is useful in relational database management systems for combining rows from two or more tables. In reference to your question, the correct answer is A. When a manager wants to see all employees in the company, regardless of whether they are associated with a particular department. This scenario entails retrieving all records from the employee table as well as any possible matches in the department table. Outer joins are particularly beneficial in this context because they allow us to see all rows from one table even if there are no corresponding rows in the joined table.
For example, in the employee and department tables scenario, an outer join would return all employees including those not assigned to any department, thus revealing employees who might require department assignments. Additionally, if using a full outer join, the query would also show any department that doesn't have employees associated with it.
In contrast, option B, C, and D would typically utilize an inner join or a type of anti-join. An inner join only returns rows with matching values in both tables, while an anti-join finds rows in one table that have no match in another. Option B suits an inner join to find employees in departments, while options C and D are concerned with existence checks and would thus likely use either an anti-join or a not exists clause.