Meaning of (+) in SQL queries

IIRC, the + is used in older versions of Oracle to indicate an outer join in the pre-ANSI SQL join syntax. In other words:

select foo,bar
from a, b
where a.id = b.id+

is the equivalent of

select foo,bar
from a left outer join b
on a.id = b.id

NOTE: this may be backwards/slightly incorrect, as I've never used the pre-ANSI SQL syntax.

It's Oracle's synonym for OUTER JOIN.

SELECT *
FROM a, b
WHERE b.id(+) = a.id

gives same result as

SELECT *
FROM a
     LEFT OUTER JOIN b
     ON b.id = a.id

The + is a short cut for OUTER JOIN, depending on which side you put it on, it indicates a LEFT or RIGHT OUTER JOIN

Check the second entry in this forum post for some examples

You use this to assure that the table you're joining doesn't reduce the amount of records returned. So it's handy when you're joining to a table that may not have a record for every key you're joining on.

For example, if you were joining a Customer and Purchase table:

To list all customers and all their purchases, do an outer join (+) on the Purchase table so customers that haven't purchased anything still show up in your report.