An inner join excludes rows from either table that don't have a matching row in the other table. An outer join provides the ability to include unmatched rows in the query results. The outer join combines the unmatched row in one of the tables with an artificial row for the other table. This artificial row has all columns set to null.
The outer join is specified in the FROM clause and has the following general format:
table-1 { LEFT | RIGHT | FULL } OUTER JOIN table-2 ON predicate-1
predicate-1 is a join predicate for the outer join. It can only reference columns from the joined tables. The LEFT, RIGHT or FULL specifiers give the type of join:
LEFT -- only unmatched rows from the left side table (table-1) are retained
RIGHT -- only unmatched rows from the right side table (table-2) are retained
FULL -- unmatched rows from both tables (table-1 and table-2) are retained
