unique (a,b)?

Hello,

I thought that a table-constraint unique (a,b) in a table like

a | b
--+--
1 | 1
2 | 2
3 | 3

would allow an insert of (1,2) as it is an unique combination. That seems to be wrong.

Is there an easy way to check unique combinations in a table definition? Or
do I have to define a special trigger?

Thanks,
Knut