Answer Posted / millerdq

The argument that no two nulls are equal does not prove
anything because it's also true that no two nulls are
different.

if null <> null ..... is false


So, one could also claim that a unique key requires that
all values are unique, given that two nulls are not unique,
it is therefore not allowed.

The fact is, ansi standard says multiple nulls are
permitted in a unique key. However, not all dbms's are
compliant. Oracle permits it; SQL Server does not.

So, best answer...it depends on the dbms

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