>This change prohibits nothing, it allows specifying something (which was a bit complicated to enforce before).
It allows specifying a constraint - that you can't have two rows with the same values even if one of the values is a NULL. That's prohibiting duplicate NULLs. The change allows you to prohibit duplicate NULLs.
Say you have a table (EmployeeName, CarID). You could do a UNIQUE constraint on those two attributes, but that would still allow:
EmployeeName | CarID
Jeff | 2
Kim | NULL
Kim | NULL
Here, "Kim" is car-less (NULL in the CarID field) twice, which makes no sense.
It allows specifying a constraint - that you can't have two rows with the same values even if one of the values is a NULL. That's prohibiting duplicate NULLs. The change allows you to prohibit duplicate NULLs.
Say you have a table (EmployeeName, CarID). You could do a UNIQUE constraint on those two attributes, but that would still allow:
EmployeeName | CarID
Jeff | 2
Kim | NULL
Kim | NULL
Here, "Kim" is car-less (NULL in the CarID field) twice, which makes no sense.
Hence the new constraint.