Q.What is a cartesian product of two tables? If table A has 6 rows and table B has 3 rows, how many rows does their cartesian product have? How is an equi-join different from a plain cartesian product?

This tests the meaning of the cartesian product, its row count, and how an equi-join refines it.

A cartesian product (also called a cross join) of two tables pairs every row of the first table with every row of the second, with no matching condition. It is produced by simply selecting from both tables:

SELECT * FROM Employee, Dept;

Number of rows: if table A has m rows and table B has n rows, the cartesian product has m x n rows. Here 6 x 3 = 18 rows. Most of these pairings are meaningless (for example, pairing a Sales employee with the Accounts department), which is why the plain cartesian product is rarely wanted on its own.

Difference from an equi-join: an equi-join takes the cartesian product and filters it, keeping only the paired rows in which the linking columns are equal (using the = operator) — typically matching a foreign key to a primary key:

SELECT Employee.Name, Dept.City
FROM Employee, Dept
WHERE Employee.Dept = Dept.DeptName;
``` …

Do not confuse the row count of a cartesian product (m x n, multiplied) with that of a UNION (roughly m + n, stacked). Remember that an equi-join is just a …