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  • 1
    Saying null is not a value (whatever that means) or that it is a marker (whatever that means) just promotes confusion & imprecision & is an impediment to clear thinking. Null is a value treated in a certain way by SQL syntax & operators. In "everyday mathematics or logic" anything can be a value, and null is a structureless everyday boring one like 7. Commented Jun 8, 2020 at 7:19
  • 1
    In standard SQL a uniqueness constraint does not enforce uniqueness if any column permits nulls. - see the thread following the question - apparently, the standard does not say this - multiple nulls are allowed for UNIQUE constraints! The thing everybody agrees on is that a NULL cannot be part of a PRIMARY KEY! Commented Jun 8, 2020 at 10:35
  • @Vérace "multiple nulls are allowed" is correct. That's what I mean when I say that uniqueness is not enforced. It's possible to have duplicate rows if you allow nulls so the rows won't necessarily be unique in the case of a nullable UNIQUE constraint. Commented Jun 8, 2020 at 17:33
  • @philipxy, Don't all values have the property of being equal to themselves? x=x is true if x is 7 but not if x is a null. Don't all values have the property of being not equal to other values? x='x is false if x is 7 but not if x is a null. Commented Jun 8, 2020 at 17:39
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    SQL = is not equals on scalars. It's like equals but it treats null differently. SQL is not distinct from is equals on scalars. Commented Jun 8, 2020 at 17:49