--- /dev/null
+#
+# 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>  </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: </td>
+<td>1. </td>
+<td>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>The version of MySQL tested (3.23.41) does not support UNION.</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>
+
+<p><hr /></p>
+<p><a href="index.html"><img src="/goback.jpg" border=0 />
+Back to the SQLite Home Page</a>
+</p>
+</body>
+</html>
+}
projects / thirdparty / sqlite.git / commitdiff
