dropping constraints

Hi,

Is there to drop a constraint when you add it separately with an alter
statement.
alter table failuretypecategory1 add constraint fk_failuretypecategory1
FOREIGN KEY (failurecategory1id) REFERENCES failurecategory1 (id) ON UPDATE CASCADE ON DELETE CASCADE;

I tried 
drop trigger <tgconstrname> on failurecategory1
and 
drop trigger <tgconstrname> on failuretypecategory1

and also

drop trigger <tgname> on failurecategory1
and 
drop trigger <tgname> on failuretypecategory1


But it kept saying 
ERROR:  DropTrigger: there is no trigger <tgname>/<tgconstrname> on
relation failuretypecategory1

What am I doing wrong.

I tried dropping the table also and when I tried to delete from a table
that it referenced above it gave the error that the table I just drop
doesn't exist.  Obviously because I just removed it but the constraint
it still there.. 

Thanks

-- 
Linh Luong