フリー問題

問題文

Two queries each return a list of product identifiers, and some identifiers appear in both lists. The lists are combined once with UNION and once with UNION ALL. How do the two results differ?

選択肢

1. The first returns the identifiers common to both queries and the second returns each identifier once, so both results leave out the rows found in one list.
2. The first returns every row of both queries and the second returns each identifier once, so the first result is the larger of the two.
3. The first returns each identifier once and the second returns every row of both queries, so the second result is at least as large as the first.
4. The first returns the identifiers of the first query only and the second returns the identifiers of both, so the second adds the rows found in the second list.