Constraint UNIQUE on a column not case sensitive

Hi,

I would like to find an efficient solution for adding/implementing a constraint UNIQUE on a VARCHAR column not case sensitive:

ALTER TABLE MyTable
ADD CONSTRAINT UNQ_MyTable_MyColumn
UNIQUE (lower(MyColumn)); -- invalid syntax

The idea is to have an index on that column, in a not case sensitive form, i.e. lower(MyColumn).

SELECT *
FROM MyTable
WHERE lower(MyColumn) = lower('...');

I don't know how to add such a constraint on MyTable except by defining a trigger on INSERT clause and checking whether lower(:NEW.MyColumn) has been already inserted in MyTable.

Is there better and more efficient way to do that?

Regards,

--
Daniel