SQL: Structured Query Language - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 44

If you ask me what is the most important thing in SQL and my
response will always be “Outer Join”. This is because it has very
high practical value. It has been used a lot in the real world
wherever SQL is utilized. I want you to cram/memorize its
definition word by word. I will first give you its definition and
explanation will follow afterwards.
Figure 2-11: Outer join: The plus (+) sign is used to join two tables.
Outer join will give you all the records from the first table and only
those records from the second table where there is a join. Positive sign
(+) will be placed with the column of the table having least number of
records, in the WHERE clause.
Outer Join will give you all the records from the first table and
only those records from the second table where there is a join. It
means that the total number of records in the result will be equal
to the number of records of the one table among the two having
no positive sign (+) attached with its column in the WHERE
clause. In other words, the way you create an Outer join is very
simple. First create a Equijoin query and then place the positive