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Comment by masklinn | Hacker News Reader

Because it’s very common for attributes to be optional but unique if specified e.g. your fleet management might have a 0-1:1 relationship between employees and cars, no car can be associated with 2 employees, an employee can’t have two cars, but any number of employee can be car-less, or car employee-less (lending pool, or cars which have not been attributed yet).

This also fits perfectly well with the normal interpretation / treatment of sql null as “not a value”.

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But an employee can't be car-less twice!

That's what this change prohibits.

> But an employee can't be car-less twice!

What.

> That's what this change prohibits.

This change prohibits nothing, it allows specifying something (which was a bit complicated to enforce before).

>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

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

Hence the new constraint.

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.

It's a 0-1:1 relationship, so you'd have columns like "name", "email", "car", etc. on the employees table, and "car" might have a unique index (which is probably an ID referencing a "cars" table).

Since employees can only have one car and since a car can only be used by one employee, an unique index is fine, but there may be 20 people that don't have a car so you don't want NULL values to be unique.

Of course there are always ways to work around this, but this is the general idea.

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