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From: Josh Kupershmidt <[email protected]>
To: Jack Douglas <[email protected]>
Cc: [email protected]
Subject: Re: boolean states
Date: Sun, 1 May 2011 19:07:10 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

On Fri, Apr 29, 2011 at 3:29 AM, Jack Douglas
<[email protected]> wrote:
> NULL is not unique to boolean, but UNKNOWN is - it would surely be wrong to
> have no mention of it at all on this page. This is because the boolean type
> is the only one used to represent truth (or logical) values. One of the
> comments from the link you provided:
>
>> What’s even more interesting is that for BOOLEAN they invented the keyword
>> UNKNOWN and the 2003 standard states “The null value of the boolean data
>> type is equivalent to the Unknown truth value.” So for BOOLEAN (and only
>> BOOLEAN AFAICT) you’re supposed to say WHERE <boolean primary> IS [NOT]
>> UNKNOWN. And in the definition of “literal”, which is supposed to “Specify a
>> non-null value”, “boolean literal” is equated to TRUE, FALSE or UNKNOWN (but
>> the latter is equivalent to a “null value” a few pages later).

Ah, OK - I had forgotten about that SQL syntax. I do agree that this sentence:
| A third state, "unknown", is represented by the SQL null value.

is particularly confusing, suggesting that "unknown" is a valid
boolean literal, on equal footing with "true" and "false".

We do document the use of IS [NOT] UNKNOWN already, see:
<http://www.postgresql.org/docs/current/interactive/functions-comparison.html;

and IMO that page is the appropriate place for such discussion. So
maybe we just need a link to that page, and should strip out the
confusing sentence about "third state" entirely? Patch attached.

Josh


Attachments:

  [application/octet-stream] boolean_unknown.patch (955B, 2-boolean_unknown.patch)
  download | inline diff:
diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index bc1ec3f..b838ee1 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -2784,10 +2784,10 @@ P <optional> <replaceable>years</>-<replaceable>months</>-<replaceable>days</> <
     <productname>PostgreSQL</productname> provides the
     standard <acronym>SQL</acronym> type <type>boolean</type>;
     see <xref linkend="datatype-boolean-table">.
-    The <type>boolean</type> type can have one of only two states:
-    <quote>true</quote> or <quote>false</quote>.  A third state,
-    <quote>unknown</quote>, is represented by the
-    <acronym>SQL</acronym> null value.
+    A non-NULL <type>boolean</type> type can have one of only two states:
+    <quote>true</quote> or <quote>false</quote>. For a list of
+    operators to use with boolean expressions, see 
+    <xref linkend="functions-comparison">.
    </para>
 
    <table id="datatype-boolean-table">

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