Re: BUG #6064: != NULL, <> NULL do not work [sec=UNCLASSIFIED]

Pilling, Michael wrote:
> Hi Kevin,
>
> Thanks for that. Point entirely taken. I think what I would add would
> be in the table 9-1 of operators, an extra column filled in only for
> =, <> and != saying Important: see difference from IS [NOT] NULL.
> Perhaps one reason I didn't pick up on this subtle issue is that IS
> NULL and IS NOT NULL are not listed in this table but they are
> comparison operators, just textual rather than symbolic ones in the
> grammar so they should be in the table. I recall specifically looking
> up what is the not equal operator in this language and only going
> forward from the table, not realising I had to read any further.
>
> I'd also add after "Do not write expression = NULL because NULL is not
> "equal to" NULL." Do not write expression != NULL or <> NULL because
> NULL is not "not equal to" NULL.  because while implied, it's not
> obvious that because = doesn't work with NULL that != doesn't either.

I have written the attached patch to mention <> NULL also returns NULL.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
new file mode 100644
index be92e6a..ddfb29a
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
***************
*** 364,371 ****
      </indexterm>
      Ordinary comparison operators yield null (signifying <quote>unknown</>),
      not true or false, when either input is null.  For example,
!     <literal>7 = NULL</> yields null.  When this behavior is not suitable,
!     use the
      <literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
  <synopsis>
  <replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>
--- 364,371 ----
      </indexterm>
      Ordinary comparison operators yield null (signifying <quote>unknown</>),
      not true or false, when either input is null.  For example,
!     <literal>7 = NULL</> yields null, as does <literal>7 <> NULL</>.  When
!     this behavior is not suitable, use the
      <literal>IS <optional> NOT </> DISTINCT FROM</literal> constructs:
  <synopsis>
  <replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>