Using Table Headers as ranges in Data Validation

[prabutr28]

Hi Members,
I have a table named "Books". My table is very simple with 3 columns.
Book.ID, Book.Name, Book.Author
All i want to do is to avoid duplicate values in Book.ID column of a table using data validation.
I simply used the formula =COUNTIF(Books[Book.ID],Books[@[Book.ID]])=1

I also used " around table names. =COUNTIF("Books[Book.ID]","Books[@[Book.ID]]")=1

But it throws formula error window...

What am i doing wrong in this?

My table structure.

Book.ID	Book.Name	Book.Author
10001	Sample Book 1	Sample Author 1
10002	Sample Book 2	Sample Author 2
10001	Sample Book 3	Sample Author 3

[#2]

Hi @prabutr28 

Try using R1C1 reference style.

=COUNTIF($A$2:A2,A2)<2

This will also allow for growth of the table.

Hope this helps.

BR,