If you are using Rails and migrations

add_index(:table, [:column1, :column2], :unique => true)

if not something like this sql

 CREATE UNIQUE INDEX index_name ON table(column1,column2);

Becker

On Fri, Apr 11, 2008 at 2:02 PM, Dave Thomas <dave / pragprog.com> wrote:
>
>  On Apr 11, 2008, at 3:47 PM, Phillip Gawlowski wrote:
>
> > Most database systems can handle that, be defining a UNIQUE constraint
> > on columns.
> >
> > However, that means that A = 1 and B = 2 won't work.
> >
>
>  Sure it will- you define the unique index to span the two columns, and then
> only non-unoque combinations of those columns will fail.
>
>  Dave
>
>