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Kim being car-less would be represented by Kim not appearing in this table at all. The design you've presented 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.

Sure.

> Here, "Kim" is car-less (NULL in the CarID field) twice, which makes no sense.

The relation makes no sense to start with, it didn’t need duplicate nulls for that.

pre postgres 15 can you solve this with two unique constraints ?

unique (EmployeeName), unique (EmployeeName, CarID)

You’d most likely solve this by making the CarID NOT NULL because there’s why would you even have a row with no useful data?

And if this is your employee table, you’d have UNIQUE (EmployeeData) and UNIQUE (CarID), and it would do what you want.

That's not quite the same, since the previous solution of unique (EmployeeName, CarID) allowed for one employee to have 2 separate cars. You have to use conditional indexes to solve this.