It is not worth memorizing because it is implementation-dependent. In Sybase derivatives (hello SQL Server), null can exist once in a unique index. A second in

It is not worth memorizing because it is implementation-dependent. In Sybase derivatives (hello SQL Server), null can exist once in a unique index. A second insertion fails. In Oracle, null is not indexed. In composite indexes, they are counted, but do not matter. My score was 11/22. I guess that I vaguely know what I'm doing.