A self-join is used to join a table to itself, and here's why the selected answers are correct:
Option B: It can be an outer join.
A self-join can indeed be either an inner or an outer join, allowing for more flexibility in how records are matched and returned, especially useful in hierarchical or sequential data scenarios.
Option E: The query must use two different aliases for the table.
When performing a self-join, aliases are necessary to distinguish between the different instances of the same table in the query.
Option F: The ON clause can be used.
In SQL, the ON clause specifies the conditions that match rows in a self-join, offering a clear and structured way to define how the join works.
Other options are not universally true:
Option A: It must be an inner join. Incorrect because, as explained, outer joins are also possible.
Option C: The ON clause must be used. Incorrect because the WHERE clause might also be used to specify the join condition.
Option D: It must be an equijoin. Incorrect because non-equijoins (like non-equality comparisons) can also be used in self-joins.