Perhaps more exactly, an unknown value is not equal to any other unknown
value, and is therefore unique.
Dennis Gearon wrote:
> the sql paradigm says NULL is 'unknown'
>
> So, you can have unique values in a column, but the unknown ones are
> ignored. this can be very handy, but is best used only in date/date
> related fields because NULLS complicate many SQL statements.
>
> P G wrote:
>
>> I have the following table with a unique constraint.
>>
>> create table testt (
>> id int,
>> aa int,
>> bb int,
>> constraint pk_testt primary key (id),
>> constraint cons_testt unique (aa,bb));
>>
>> Let's say I insert the following:
>>
>> insert into testt values (1,2,null);
>> insert into testt values (2,2,null);
>>
>> I thought this should fail because of the unique
>> constraint, but it doesn't. How does PostgreSQL view
>> the null values in this constraint? Will someone
>> explain why I am allowed to make this insertion?
>>
>> TIA.
>>
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