CROSS JOIN   This type of join of two tables produces all possible combinations of rows from the two tables. The size of the result set is the number of rows in the first table multiplied by the number of rows in the second table. A cross join is also called a cross product or Cartesian product. You cannot use an ON clause with a cross join.

KEY JOIN   This type of join condition uses the foreign key relationships between the tables. Key join is the default when the JOIN keyword is used without specifying a join type (such a INNER, OUTER, and so on) and there is no ON clause.

NATURAL JOIN   This join is automatically generated based on columns having the same name.

Join using an ON clause   This type of join results from explicit specification of the join condition in an ON clause. When used with a key join or natural join, the join condition contains both the generated join condition and the explicit join condition. When used with the keyword JOIN without the keywords KEY or NATURAL, there is no generated join condition. See Explicit join conditions (the ON clause).