A type of join that returns records from one table even when no matching values exist in the other table. The three types of outer join are left, right, and full outer join. A left outer join returns all records from the table on the left side of the join expression, even if no matching values exist in the table on the right side. A right outer join returns all records from the table on the right side of the join expression, even if no matching values exist in the table on the left side. For example, joining customers and orders tables on customerID with the customers table on the left side of the expression returns a result set that contains all customer records, including customers who have no orders. A full outer join is the union of the result sets of both left and right outer joins.