What is Included Column?

Hi All,
I saw new concept of included column in sql 2005, but when I was trying to create index.
I just wanted to know what is it and I have a small question for this also.
I have a table which contains 20 columns. I need a unique index on 18 numbers of columns
which throws me an error “maximum 16 columns can be possible for an index”.
Then I tried with this included column concepts and its possible now...
but when I am trying to insert data. Its not checking my uniqueness of data..
Why So..??
Thanks,
Sandy.

Creating unique index irrespective of using INCLUDE COLUMNS it behaves in manner below
BOOK_TABLE
TITLE NAME PUBLISHER
TT1 HELLO PUB1
TT1 HELLO PUB2
The second row is allowed since the it is unique as complete row, hope this makes things clearer for you.
-Sat

Included columns are not part of the index itself, so they do not contribute to the uniqueness.
As far as understand it, you can INCLUDE columns with a key index in case you often filter/join on the combination of key and additional column.
Wow - 16 columns to be unique. If you are sure you need that many, you could create an insert/update trigger to check for uniqueness. Easiest if you also have an identity column:
CREATE TRIGGER trigCheckUnique_IU ON MyTable FOR INSERT, UPDATE
AS
IF EXISTS
(SELECT i.key FROM inserted i
WHERE EXISTS
(SELECT t.key FROM MyTable t WHERE t.key <> i.key AND t.col1 = i.col1 AND .........etc))

BEGIN

END