Joins are based on the relational operator product, a direct analogue of an operator in set
theory known as the Cartesian product . It is denoted by a symbol cross (x). Product takes
two relations as input and produces as output one relation composed of all the possible
combinations of input tuples/rows. Product is a little-used operator in practice because of
its potential for generating an 'information explosion'.
Syntax of the product operator is given below:
PRODUCT <table 1> WITH <table 2><result table>
PRODUCT Teacher WITH Subject R
In the following figure we have two relation Teacher and Subject.
The output of this product have all combination of tuples/rows of both relations/ tables. We
get a relation of six tuples with six attributes.