Re: NULL as a (pseudo-)value not described?

On 09/11/14 10:03, David Johnston wrote: 

On Thu, Sep 11, 2014 at 9:24 AM, John Lumby <johnlumby@hotmail.com> wrote:

​As I mentioned before the fact that null is a literal/constant is assumed. My point here is that given that assumption the referenced section explains that "SET col = null" is valid.  SET col = <expression>, <constants> are <expressions>, null is a valid <constant>, ergo SET col = null is valid.

Ah,  ok.    Well,   I would say that NULL is not a constant,
since a constant has a well-defined value and can be used
to represent that value in any expression,  neither of which
are true of NULL.   However see my last para.

The documentation does not describe all possible valid constants - though I admit given the special nature of NULL it probably should do so in this instance.  The trick is avoiding adding comments pertaining to NULL all over the documentation (see below) and confusing the underlying normal non-null usage.  Thus, right now, only when NULL behavior is important does it get addressed.  SET col = NULL is no different then SET col  = 'a string' so why make it seem like a special case by pointing out the "obvious"?

Even if people are not positive simply trying what you wrote is easy and in the absence of any error it would become obvious that NULL is valid in UPDATE/SET.

The larger problem is people thinking "NULL = NULL" returns true or "NULL = 'some other literal'" returns FALSE: i.e., that NULL is never special but is just another literal.

Actually I have seen mailings where someone is asking how to set a column value to NULL
so I'd say it is not obvious.      A reference manual should preferably avoid
assuming anything about what it is documenting.

Well,   I would assume that one or other of the SQL ANSI standards documents NULL
and (provided postgresql confirms to that standard) we can simply copy or rephrase
that text.   However,  I personally am not a standards expert, and after a short
and futile search on on the web I can't find any accessible html/pdf format of any SQL ANSI standard.
I assume someone more knowledgeable on standards could find it.
But if I had to take a shot at it,  I would add a description of NULL at the bottom of section 4.2
following the text :
   "In addition to this list, there are a number of constructs that can be classified
     as an expression but do not follow any general syntax rules.
     These generally have the semantics of a function or operator and are explained
     in the appropriate location in Chapter 9. An example is the IS NULL clause."

something along these lines :

<start  addition>
     Another example is the NULL token.     This is not a value but can be included in certain commands
     where a value can be assigned to a column,  such as INSERT (the VALUES clause)
    and UPDATE (the SET clause),      in the same position as a value would be placed,
    provided the column is not defined as NOT NULL.    Its meaning is "no value".
     It is not equal to any other value,   and not unequal to any other value,
    and not equal to another NULL,  and not unequal to another NULL.
     NULL may not be stated as an explicit default value for a column,
      but if no default value is defined for a column which is not defined as NOT NULL,
      then its default value is NULL.    In this case,  NULL has the sense of "not set".
      <is the next true?  I am not somewhere to verify ...?>
       It cannot be used in an expression other than when it occurs alone without any associated operator or function.
      </is the above true?  I am not somewhere to verify ...?>
  </end addition>