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>