Some DBMS allow multiple NULL values in a unique index, some don't.

The SQL standard only covers unique constraints, not unique indexes. A unique constraint may contain multiple null values

create table foo 
(
  col1   integer,
  col2   integer
);

create unique index idx_foo 
   on foo (col1,col2);

insert into foo (col1, col2) values (1, null);
insert into foo (col1, col2) values (1, null);  -- will fail on some DBMS, will work on others

Back to the SQL Feature Comparison