Re: Is PRIMARY KEY the same as UNIQUE NOT NULL?

Harald Fuchs <hari.fuchs@gmail.com> writes:
> Ben Leslie <benno@benno.id.au> writes:
>> "Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL"
>>
>> I wanted to clarify if that was, technically, true.

> Yes, but see below.

>> "identifying a set of columns as primary key also provides metadata
>> about the design of the schema, as a primary key implies that other
>> tables can rely on this set of columns as a unique identifier for
>> rows."

Yeah.  The extra metadata has several other effects.  Perhaps it would be
better to reword this sentence to make it clear that PRIMARY KEY is
equivalent to UNIQUE+NOTNULL in terms of the data constraint that it
enforces, without implying that there is no other difference.  I'm not
sure about a short and clear expression of that though ...

            regards, tom lane