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Add documentation on NULL handling in SQLite versus other SQL database 

engines. (CVS 741)

FossilOrigin-Name: dd8867fc479891ed8fa36c5cf2359e49c6754a8c
This commit is contained in:
drh 2002-09-02 14:11:02 +00:00
parent 0e1cfb8574
commit 96f4531077
5 changed files with 278 additions and 10 deletions
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6
main.mk
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@ -308,6 +308,9 @@ quickstart.html: $(TOP)/www/quickstart.tcl
fileformat.html: $(TOP)/www/fileformat.tcl
tclsh $(TOP)/www/fileformat.tcl >fileformat.html
nulls.html: $(TOP)/www/nulls.tcl
tclsh $(TOP)/www/nulls.tcl >nulls.html
# Files to be published on the website.
#
@ -332,7 +335,8 @@ DOC = \
omitted.html \
datatypes.html \
quickstart.html \
fileformat.html
fileformat.html \
nulls.html
doc: $(DOC)
mkdir -p doc

15
manifest
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@ -1,5 +1,5 @@
C Detect\swhen\sthe\stest\sscripts\sare\sbeing\srun\sas\sroot\sand\sissue\san\sappropriate\nerror\smessage.\s(CVS\s740)
D 2002-09-02T12:14:51
C Add\sdocumentation\son\sNULL\shandling\sin\sSQLite\sversus\sother\sSQL\sdatabase\nengines.\s(CVS\s741)
D 2002-09-02T14:11:03
F Makefile.in 420fada882179cb72ffd07313f3fd693f9f06640
F Makefile.linux-gcc b86a99c493a5bfb402d1d9178dcdc4bd4b32f906
F README f1de682fbbd94899d50aca13d387d1b3fd3be2dd
@ -14,7 +14,7 @@ F doc/report1.txt a031aaf37b185e4fa540223cb516d3bccec7eeac
F install-sh 9d4de14ab9fb0facae2f48780b874848cbf2f895
F libtool c56e618713c9510a103bda6b95f3ea3900dcacd6
F ltmain.sh e9ed72eb1d690f447c13945eaf69e28af531eda1
F main.mk 8e34134476c039c89bb404f2712bcbb2f044bdfe
F main.mk 14a6933b738143bf939e9c04c59163d738239fae
F publish.sh a7a8d23e6525bd25d4f5ba9b0fc6edc107d94050
F spec.template 238f7db425a78dc1bb7682e56e3834c7270a3f5e
F sqlite.1 83f4a9d37bdf2b7ef079a82d54eaf2e3509ee6ea
@ -140,9 +140,10 @@ F www/dynload.tcl 02eb8273aa78cfa9070dd4501dca937fb22b466c
F www/faq.tcl 207d3e31597c63ed3bbecd58aaeaa38c53d39dd4
F www/fileformat.tcl a4b5c2c6e89b7d42d09f97fd4d7bbd39cbf24936
F www/formatchng.tcl b4449e065d2da38b6563bdf12cf46cfe1d4d765e
F www/index.tcl 33881038e9664a36e56df3b80ef0828594c8dcd9
F www/index.tcl 190f767c4b0bf44f75bf4af023d644c3e7a1e579
F www/lang.tcl bf9d830aa0042718ae157e25fc7539f5378c46ff
F www/mingw.tcl f1c7c0a7f53387dd9bb4f8c7e8571b7561510ebc
F www/nulls.tcl 29497dac2bc5b437aa7e2e94577dad4d8933ed26
F www/omitted.tcl 118062f40a203fcb88b8d68ef1d7c0073ac191ec
F www/opcode.tcl 33c5f2061a05c5d227c72b84c080b3bf74c74f8b
F www/quickstart.tcl fde79aa2de20074842b60f780800cdeee6a5dec2
@ -150,7 +151,7 @@ F www/speed.tcl a20a792738475b68756ea7a19321600f23d1d803
F www/sqlite.tcl ae3dcfb077e53833b59d4fcc94d8a12c50a44098
F www/tclsqlite.tcl 1db15abeb446aad0caf0b95b8b9579720e4ea331
F www/vdbe.tcl 2013852c27a02a091d39a766bc87cff329f21218
P 8c2a0836980341faa479cfe6c716409e6057367d
R d69d74396bd3d8fa6110e1bb4d145b9c
P 9ca2c507704f85446d873d5e8429554f2ad4df9e
R a486780ea79cbf7fa41ff4e723ebd4b5
U drh
Z 5b7e102d83ef9c4dd3f46fe8bd1002c5
Z 27d152bf1210ee491944a9449323bf4b

2
manifest.uuid
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@ -1 +1 @@
9ca2c507704f85446d873d5e8429554f2ad4df9e
dd8867fc479891ed8fa36c5cf2359e49c6754a8c

4
www/index.tcl
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@ -1,7 +1,7 @@
#
# Run this TCL script to generate HTML for the index.html file.
#
set rcsid {$Id: index.tcl,v 1.67 2002/08/18 19:09:24 drh Exp $}
set rcsid {$Id: index.tcl,v 1.68 2002/09/02 14:11:04 drh Exp $}
puts {<html>
<head><title>SQLite: An Embeddable SQL Database Engine</title></head>
@ -139,6 +139,8 @@ puts {<h2>Documentation</h2>
<li>SQLite is <a href="datatypes.html">typeless</a>.
<li>The <a href="lang.html">SQL Language</a> subset understood by SQLite.</li>
<li>The <a href="c_interface.html">C/C++ Interface</a>.</li>
<li>The <a href="nulls.html">NULL handling</a> in SQLite versus
other SQL database engines.</li>
<li>The <a href="tclsqlite.html">Tcl Binding</a> to SQLite.</li>
<li>The <a href="arch.html">Architecture of the SQLite Library</a> describes
how the library is put together.</li>

261
www/nulls.tcl Normal file
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@ -0,0 +1,261 @@
#
# Run this script to generated a nulls.html output file
#
set rcsid {$Id: nulls.tcl,v 1.1 2002/09/02 14:11:04 drh Exp $}
puts {<html>
<head>
<title>NULL Handling In SQLite Versus Other Database Engines</title>
</head>
<body bgcolor="white">
<h1 align="center">
NULL Handling in SQLite Versus Other Database Engines
</h1>
}
puts "<p align=\"center\">
(This page was last modified on [lrange $rcsid 3 4] UTC)
</p>"
puts {
<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.
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
expriments 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 index. 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>
<p>
The following table shows the results of the NULL handling experiments.
</p>
<table border=1 cellpadding=5 width="100%">
<tr><th>&nbsp&nbsp;</th>
<th>SQLite</th>
<th>PostgreSQL</th>
<th>Oracle</th>
<th>Informix</th>
<th>DB2</th>
<th>MS-SQL</th>
<th>MySQL</th>
<th>OCELOT</th>
<th>Firebird</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>
<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>
<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 index</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="#a9c7a9">Yes</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>
</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>
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
<td valign="center" align="center" bgcolor="#aaaad2">(Note 1)</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="#aaaad2">(Note 3)</td>
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
<td valign="center" align="center" bgcolor="#aaaad2">(Note 1)</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>
<td valign="center" align="center" bgcolor="#a9c7a9">Yes</td>
<td valign="center" align="center" bgcolor="#aaaad2">(Note 2)</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>
<td valign="center" align="center" bgcolor="#c7a9a9">No</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="#c7a9a9">No</td>
<td valign="center" align="center" bgcolor="#c7a9a9">No</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=3>Notes:&nbsp;&nbsp;</td>
<td>1.&nbsp;</td>
<td>Firebird omits all NULLs from SELECT DISTINCT and from UNION.</td>
</tr>
<tr><td>2.&nbsp;</td>
<td>Test data unavailable.</td></tr>
<tr><td>3.&nbsp;</td>
<td>The version of MySQL tested (3.23.41) does not support UNION.</td></tr>
</table>
<br clear="both">
<p>&nbsp;</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>
<p><hr /></p>
<p><a href="index.html"><img src="/goback.jpg" border=0 />
Back to the SQLite Home Page</a>
</p>
</body>
</html>
}