Comments - Getting Started with Indexes
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"Unique Index - A Unique Index must be unique, but it can be null. So each key value identifies only one record, but not each record needs to be represented."
That implies that a unique index on a nullable column would need a table scan to do a WHERE c IS NULL query. I think it was meant to say that a UNIQUE index can include a nullable column, to for example allow UNIQUE indexes on (firstname, lastname) (where firstname is not nullable, and lastname is nullable) also work for rows that have a NULL lastname. Then also include the example that ('john',NULL) is not unique, so can occur many times.
Furthermore, I consider that (almost) all occurrences of 'record' and 'field' should be replaced by 'row' and 'column'.