So I have two columns of Data Validations:

Column A we'll call Cost Centers
Column B we'll call Accounts

If somebody puts in a Cost Center for a specific department which we
will call Advertising, then a different list shows up in Column B of
only Advertising Accts. I use the following formula in Column B for
my Data Validation:

=IF(ISERROR(VLOOKUP(A5,advertising,1,0)),accounts, advaccts) where
"accounts" is all accounts and "advaccts" is only advertising
accounts.

That works ok. I'm sure that is not the best way of doing it; but
here is my problem.

If I don't put anything in Column A, users can type anything into
Column B and there is no Data Validation. I was hoping that it would
use the "accounts" list as validation if nothing was entered into
Column A.

Thoughts on how to keep data validation on column B if nothing is put
into Column A?