Question

To answer the following questions, refer to the tables in the JustLee Books database.

Which of the following queries creates a Cartesian join?
SELECT title, authorid
FROM books, bookauthor;
SELECT title, name
FROM books CROSS JOIN publisher;
SELECT title, gift
FROM books NATURAL JOIN promotion;
all of the above

Answer 1

A Cartesian join is created when there is no explicit join condition between tables or the CROSS JOIN keyword is used. In the given options, the first two queries create Cartesian joins, while the last uses a NATURAL JOIN, which is different.

Step-by-step explanation:

The question is asking about the type of join created by different SQL queries when querying a database. A Cartesian join, or cross join, is a join operation that returns the Cartesian product of two or more tables. This means it combines every row from one table with every row from the other table(s), which can result in a very large number of rows in the result set if the tables have many rows.

Looking at the provided queries, the following query:
SELECT title, authorid FROM books, bookauthor;
creates a Cartesian join because there is no explicit join condition between the books and bookauthor tables. Whereas,
SELECT title, name FROM books CROSS JOIN publisher;
explicitly uses the CROSS JOIN keyword, thereby also creating a Cartesian join. The last query,
SELECT title, gift FROM books NATURAL JOIN promotion;
uses a NATURAL JOIN, which is different from a Cartesian join because it implicitly joins the tables using all columns with the same names in both tables.

Therefore, only the first two queries in the list create Cartesian joins.