Oracle External connections
(1) Left outer connection (the table on the left is unrestricted)
(2) Right outer connection (no restriction on the right table)
(3) Full-outer connection (no restriction on both the left and right tables)
Outer JOIN (Outer join)
Outer join returns a row for each join that satisfies the first (top) input and the second (bottom) input. It also returns any row in the first input that does not have a matching row in the second input. The outer connection is divided into three kinds: Left outer connection, right outer connection, full outer connection. Corresponds to Sql:left/right/full OUTER JOIN. Usually we omit the outer keyword. Written as: Left/right/full JOIN.
A table is the base table for both the left and right outer joins, and the contents of the table are all displayed, followed by two tables that match the contents. If the data in the base table is not recorded in another table. Then the columns in the associated result set row are displayed as null values (NULL).
For outer joins, you can also use "(+)" to represent them. Some things to note about using (+):
1. The (+) operator can only appear in the WHERE clause and cannot be used in conjunction with the outer join syntax.
2. When an outer join is performed using the (+) operator, if more than one condition is included in the WHERE clause, the (+) operator must be included in all conditions
3. The (+) operator applies only to columns, not to expressions.
4. The (+) operator cannot be used with the OR and in operators.
5. The (+) operator can only be used to implement left outer and right outer joins, not for full outer joins.
Http://blog.chinaunix.net/uid-21187846-id-3288525.html
http://blog.itpub.net/519536/viewspace-563019/
Oracle left JOIN, right connection, full outer join, (+) sign action