Specifying the Join Type When Creating a Join Index

The join type is always FULL OUTER, the keyword OUTER being optional.

You also need to do one of the following:

If you are joining equivalent columns with the same name from two tables, you specify that it is a NATURAL JOIN.
If you are joining columns based on keys, you must also have specified the relationship in the underlying tables as a FOREIGN KEY that references a PRIMARY KEY.
If you are joining equivalent values (an equijoin) in columns from two tables, you specify an ON clause.

These rules conform to ANSI syntax requirements.