inner join :any join that combines the records form two tables using some condition
natural join:an inner join that leaves off "on" clause.it only works if you are joining
two tables that have same column name.
equijoin and non-equijoin : both are inner join.the equijoin returns rows that are equal,
and the non-equijoin returns any rows that are not equal.
cross join :return every row from one table crossed with every row from the second table.known by many other names including cartesian join.
comma join: the same thing as a cross join, except a comma is used instead of the keywords cross join