330 lines
13 KiB
Tcl
330 lines
13 KiB
Tcl
#
|
|
# Run this script to generated a nulls.html output file
|
|
#
|
|
set rcsid {$Id: nulls.tcl,v 1.8 2004/10/10 17:24:55 drh Exp $}
|
|
source common.tcl
|
|
header {NULL Handling in SQLite}
|
|
puts {
|
|
<h2>NULL Handling in SQLite Versus Other Database Engines</h2>
|
|
|
|
<p>
|
|
The goal is
|
|
to make SQLite handle NULLs in a standards-compliant way.
|
|
But the descriptions in the SQL standards on how to handle
|
|
NULLs seem ambiguous.
|
|
It is not clear from the standards documents exactly how NULLs should
|
|
be handled in all circumstances.
|
|
</p>
|
|
|
|
<p>
|
|
So instead of going by the standards documents, various popular
|
|
SQL engines were tested to see how they handle NULLs. The idea
|
|
was to make SQLite work like all the other engines.
|
|
A SQL test script was developed and run by volunteers on various
|
|
SQL RDBMSes and the results of those tests were used to deduce
|
|
how each engine processed NULL values.
|
|
The original tests were run in May of 2002.
|
|
A copy of the test script is found at the end of this document.
|
|
</p>
|
|
|
|
<p>
|
|
SQLite was originally coded in such a way that the answer to
|
|
all questions in the chart below would be "Yes". But the
|
|
experiments run on other SQL engines showed that none of them
|
|
worked this way. So SQLite was modified to work the same as
|
|
Oracle, PostgreSQL, and DB2. This involved making NULLs
|
|
indistinct for the purposes of the SELECT DISTINCT statement and
|
|
for the UNION operator in a SELECT. NULLs are still distinct
|
|
in a UNIQUE column. This seems somewhat arbitrary, but the desire
|
|
to be compatible with other engines outweighted that objection.
|
|
</p>
|
|
|
|
<p>
|
|
It is possible to make SQLite treat NULLs as distinct for the
|
|
purposes of the SELECT DISTINCT and UNION. To do so, one should
|
|
change the value of the NULL_ALWAYS_DISTINCT #define in the
|
|
<tt>sqliteInt.h</tt> source file and recompile.
|
|
</p>
|
|
|
|
<blockquote>
|
|
<p>
|
|
<i>Update 2003-07-13:</i>
|
|
Since this document was originally written some of the database engines
|
|
tested have been updated and users have been kind enough to send in
|
|
corrections to the chart below. The original data showed a wide variety
|
|
of behaviors, but over time the range of behaviors has converged toward
|
|
the PostgreSQL/Oracle model. The only significant difference
|
|
is that Informix and MS-SQL both threat NULLs as
|
|
indistinct in a UNIQUE column.
|
|
</p>
|
|
|
|
<p>
|
|
The fact that NULLs are distinct for UNIQUE columns but are indistinct for
|
|
SELECT DISTINCT and UNION continues to be puzzling. It seems that NULLs
|
|
should be either distinct everywhere or nowhere. And the SQL standards
|
|
documents suggest that NULLs should be distinct everywhere. Yet as of
|
|
this writing, no SQL engine tested treats NULLs as distinct in a SELECT
|
|
DISTINCT statement or in a UNION.
|
|
</p>
|
|
</blockquote>
|
|
|
|
|
|
<p>
|
|
The following table shows the results of the NULL handling experiments.
|
|
</p>
|
|
|
|
<table border=1 cellpadding=3 width="100%">
|
|
<tr><th>  </th>
|
|
<th>SQLite</th>
|
|
<th>PostgreSQL</th>
|
|
<th>Oracle</th>
|
|
<th>Informix</th>
|
|
<th>DB2</th>
|
|
<th>MS-SQL</th>
|
|
<th>OCELOT</th>
|
|
</tr>
|
|
|
|
<tr><td>Adding anything to null gives null</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>Multiplying null by zero gives null</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in a UNIQUE column</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#aaaad2">(Note 4)</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in SELECT DISTINCT</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in a UNION</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
</tr>
|
|
<tr><td>"CASE WHEN null THEN 1 ELSE 0 END" is 0?</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>"null OR true" is true</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>"not (null AND false)" is true</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<table border=1 cellpadding=3 width="100%">
|
|
<tr><th>  </th>
|
|
<th>MySQL<br>3.23.41</th>
|
|
<th>MySQL<br>4.0.16</th>
|
|
<th>Firebird</th>
|
|
<th>SQL<br>Anywhere</th>
|
|
<th>Borland<br>Interbase</th>
|
|
</tr>
|
|
|
|
<tr><td>Adding anything to null gives null</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>Multiplying null by zero gives null</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in a UNIQUE column</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#aaaad2">(Note 4)</td>
|
|
<td valign="center" align="center" bgcolor="#aaaad2">(Note 4)</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in SELECT DISTINCT</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No (Note 1)</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
</tr>
|
|
<tr><td>nulls are distinct in a UNION</td>
|
|
<td valign="center" align="center" bgcolor="#aaaad2">(Note 3)</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No (Note 1)</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
</tr>
|
|
<tr><td>"CASE WHEN null THEN 1 ELSE 0 END" is 0?</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#aaaad2">(Note 5)</td>
|
|
</tr>
|
|
<tr><td>"null OR true" is true</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
<tr><td>"not (null AND false)" is true</td>
|
|
<td valign="center" align="center" bgcolor="#c7a9a9">No</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<table border=0 align="right" cellpadding=0 cellspacing=0>
|
|
<tr>
|
|
<td valign="top" rowspan=5>Notes: </td>
|
|
<td>1. </td>
|
|
<td>Older versions of firebird omits all NULLs from SELECT DISTINCT
|
|
and from UNION.</td>
|
|
</tr>
|
|
<tr><td>2. </td>
|
|
<td>Test data unavailable.</td>
|
|
</tr>
|
|
<tr><td>3. </td>
|
|
<td>MySQL version 3.23.41 does not support UNION.</td>
|
|
</tr>
|
|
<tr><td>4. </td>
|
|
<td>DB2, SQL Anywhere, and Borland Interbase
|
|
do not allow NULLs in a UNIQUE column.</td>
|
|
</tr>
|
|
<tr><td>5. </td>
|
|
<td>Borland Interbase does not support CASE expressions.</td>
|
|
</tr>
|
|
</table>
|
|
<br clear="both">
|
|
|
|
<p> </p>
|
|
<p>
|
|
The following script was used to gather information for the table
|
|
above.
|
|
</p>
|
|
|
|
<pre>
|
|
-- I have about decided that SQL's treatment of NULLs is capricious and cannot be
|
|
-- deduced by logic. It must be discovered by experiment. To that end, I have
|
|
-- prepared the following script to test how various SQL databases deal with NULL.
|
|
-- My aim is to use the information gather from this script to make SQLite as much
|
|
-- like other databases as possible.
|
|
--
|
|
-- If you could please run this script in your database engine and mail the results
|
|
-- to me at drh@hwaci.com, that will be a big help. Please be sure to identify the
|
|
-- database engine you use for this test. Thanks.
|
|
--
|
|
-- If you have to change anything to get this script to run with your database
|
|
-- engine, please send your revised script together with your results.
|
|
--
|
|
|
|
-- Create a test table with data
|
|
create table t1(a int, b int, c int);
|
|
insert into t1 values(1,0,0);
|
|
insert into t1 values(2,0,1);
|
|
insert into t1 values(3,1,0);
|
|
insert into t1 values(4,1,1);
|
|
insert into t1 values(5,null,0);
|
|
insert into t1 values(6,null,1);
|
|
insert into t1 values(7,null,null);
|
|
|
|
-- Check to see what CASE does with NULLs in its test expressions
|
|
select a, case when b<>0 then 1 else 0 end from t1;
|
|
select a+10, case when not b<>0 then 1 else 0 end from t1;
|
|
select a+20, case when b<>0 and c<>0 then 1 else 0 end from t1;
|
|
select a+30, case when not (b<>0 and c<>0) then 1 else 0 end from t1;
|
|
select a+40, case when b<>0 or c<>0 then 1 else 0 end from t1;
|
|
select a+50, case when not (b<>0 or c<>0) then 1 else 0 end from t1;
|
|
select a+60, case b when c then 1 else 0 end from t1;
|
|
select a+70, case c when b then 1 else 0 end from t1;
|
|
|
|
-- What happens when you multiple a NULL by zero?
|
|
select a+80, b*0 from t1;
|
|
select a+90, b*c from t1;
|
|
|
|
-- What happens to NULL for other operators?
|
|
select a+100, b+c from t1;
|
|
|
|
-- Test the treatment of aggregate operators
|
|
select count(*), count(b), sum(b), avg(b), min(b), max(b) from t1;
|
|
|
|
-- Check the behavior of NULLs in WHERE clauses
|
|
select a+110 from t1 where b<10;
|
|
select a+120 from t1 where not b>10;
|
|
select a+130 from t1 where b<10 OR c=1;
|
|
select a+140 from t1 where b<10 AND c=1;
|
|
select a+150 from t1 where not (b<10 AND c=1);
|
|
select a+160 from t1 where not (c=1 AND b<10);
|
|
|
|
-- Check the behavior of NULLs in a DISTINCT query
|
|
select distinct b from t1;
|
|
|
|
-- Check the behavior of NULLs in a UNION query
|
|
select b from t1 union select b from t1;
|
|
|
|
-- Create a new table with a unique column. Check to see if NULLs are considered
|
|
-- to be distinct.
|
|
create table t2(a int, b int unique);
|
|
insert into t2 values(1,1);
|
|
insert into t2 values(2,null);
|
|
insert into t2 values(3,null);
|
|
select * from t2;
|
|
|
|
drop table t1;
|
|
drop table t2;
|
|
</pre>
|
|
}
|
|
|
|
footer $rcsid
|