Create unique index or constraint on part of a column

[Ruben Blanco <rubenblan@xxxxxxxxx>]

Hi:

Is there anyway to create a unique index or constraint on part of a column? 

Something like this, but something that works ;-)

  ALTER TABLE invoices 
  ADD CONSTRAINT cons UNIQUE (EXTRACT(YEAR FROM invoice_date), innvoice_number);

Thanks for any help.
Ruben,