Which statement best describes an outer join?

• A. Includes all rows from both tables.
• B. Includes all rows from the first table and all rows from the second table.
• C. Includes only rows from the first table that have matches in the second.
• D. Includes all rows from one table and only those rows from the other table that match the join field in the first table.

Answer: (D)

Outer joins preserve rows from one table while bringing in matching rows from the other, and fill in NULLs when there is no match. The statement describes keeping all rows from one side and only the matching rows from the other, with NULLs for the non-matching side. That is exactly how a left outer join (or, symmetrically, a right outer join if you preserve the other table) works: you keep every row from the preserved table and attach matching rows from the joined table, or NULLs when there are none. For example, a left outer join from customers to orders on customer_id returns every customer, with their orders where they exist, and NULLs for customers without orders. In contrast, including all rows from both tables would describe a full outer join, and including only rows with matches in both tables would describe an inner join.