=COUNTIF(A:A,A2)=0

where a2 is the DV cell

--
HTH

Bob Phillips

(replace somewhere in email address with gmail if mailing direct)

"Luke" wrote in message
...
Hi

I want to create a formula for a data validation.

I must not be able to enter a "ticket number" that already exists in my
database in a specific column.

Regards,

Luke