Cartesian product of two sets of records A and B returns a set C such that C contains all ordered pairs (a, b) where a belongs to A and b belongs to B.
This definition can be easily generalized to the case of several sets.
Main idea behind cartesian product (or cartesian join, or cross join) is that it returns all possible combinations of rows from all tables specified in the query.
The CARTESIAN JOIN or CROSS JOIN returns the Cartesian product of the sets of records from the two or more joined tables. Thus, it equates to an inner join where the join-condition always evaluates to True or where the join-condition is absent from the statement.
The basic syntax of CARTESIAN JOIN or CROSS JOIN is as follows:
SELECT table1.column1, table2.column2...
FROM table1, table2 [, table3 ]