Examine the structure of the customers table: CUSTNO is the primary key in the table. You want to find out if any customers' details have been entered more than once using different CUSTNO, by…

A self-join joins the CUSTOMERS table to itself on matching name columns while filtering for rows where CUSTNO values differ, directly producing a result set of all name pairs entered under different customer numbers.

A subquery using GROUP BY on the name columns with HAVING COUNT(*) > 1 identifies which names appear more than once, and the outer query then retrieves all matching rows including their distinct CUSTNO values.

A full outer join returns all rows from both sides of the join including non-matching rows, which adds irrelevant data and does not improve duplicate detection beyond what a simple self-join provides.

A left outer join preserves all rows from the left table regardless of whether they match, introducing unnecessary rows that complicate rather than simplify finding duplicates.

A right outer join preserves all rows from the right table regardless of match status, which similarly adds unwanted complexity without offering any advantage over a standard self-join for this use case.