Consider the following two tables and four queries in SQL. Book ($\underline{\text{isbn}}$, bname), Stock ($\underline{\text{isbn}}$, copies) Query 1:SELECT B.isbn, S.copies FROM Book B INNER JOIN Stock S ON B.isbn = S.isbn;Query 2:SELECT B.isbn, S.copies FROM Book B LEFT OUTER JOIN Stock S ON B.isbn = S.isbn;Query 3:SELECT B.isbn, S.copies FROM Book B RIGHT OUTER JOIN Stock S ON B.isbn = S.isbn;Query 4:SELECT B.isbn, S.copies FROM Book B FULL OUTER JOIN Stock S ON B.isbn = S.isbn;Which one of the queries above is certain to have an output that is a superset of the outputs of the other three queries?

Let us analyze the output tuples produced by each query:

1. Query 1 (INNER JOIN): Contains all rows where B.isbn = S.isbn in both tables: (B.isbn, S.copies).

2. Query 2 (LEFT OUTER JOIN): Contains all rows from the INNER JOIN plus rows for Book entries with no matching isbn in Stock, with S.copies set to NULL.

3. Query 3 (RIGHT OUTER JOIN): Contains all rows from the INNER JOIN plus rows for Stock entries with no matching isbn in Book, with B.isbn set to NULL.

4. Query 4 (FULL OUTER JOIN): Combines the results of both LEFT OUTER JOIN and RIGHT OUTER JOIN. It includes:

   • All matching tuples (the INNER JOIN result),

   • Unmatched rows from Book (with S.copies = NULL),

   • Unmatched rows from Stock (with B.isbn = NULL).

Therefore, the output set of Query 4 contains all tuples present in Query 1, Query 2, and Query 3, making it a guaranteed superset of the other three queries.