GATE CS 2018 Set 1 — Question 22
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Databases → SQL → SQL Joins
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Question
Consider the following two tables and four queries in SQL.Book (isbn, bname), Stock (isbn, copies)Query 1:
Query 2:
Query 3:
Query 4:
Which one of the queries above is certain to have an output that is a superset of the outputs of the other three queries?
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?
Correct answer
(D) Query 4
Solution
In SQL joins:
and .
Thus, (Query 4) is always a superset of and .
- INNER JOIN returns only the rows that have matching values in both tables.
- LEFT OUTER JOIN returns all rows from the left table, and the matched rows from the right table. If no match, NULL is returned for right table columns.
- RIGHT OUTER JOIN returns all rows from the right table, and the matched rows from the left table. If no match, NULL is returned for left table columns.
- FULL OUTER JOIN returns all rows when there is a match in either left or right table records. It effectively combines the results of both LEFT and RIGHT outer joins.
and .
Thus, (Query 4) is always a superset of and .
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