1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html><head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title>NULL Handling in SQLite</title>
<style type="text/css">
body {
margin: auto;
font-family: Verdana, sans-serif;
padding: 8px 1%;
}
a { color: #044a64 }
a:visited { color: #734559 }
.logo { position:absolute; margin:3px; }
.tagline {
float:right;
text-align:right;
font-style:italic;
width:300px;
margin:12px;
margin-top:58px;
}
.menubar {
clear: both;
border-radius: 8px;
background: #044a64;
padding: 0px;
margin: 0px;
cell-spacing: 0px;
}
.toolbar {
text-align: center;
line-height: 1.6em;
margin: 0;
padding: 0px 8px;
}
.toolbar a { color: white; text-decoration: none; padding: 6px 12px; }
.toolbar a:visited { color: white; }
.toolbar a:hover { color: #044a64; background: white; }
.content { margin: 5%; }
.content dt { font-weight:bold; }
.content dd { margin-bottom: 25px; margin-left:20%; }
.content ul { padding:0px; padding-left: 15px; margin:0px; }
/* Things for "fancyformat" documents start here. */
.fancy img+p {font-style:italic}
.fancy .codeblock i { color: darkblue; }
.fancy h1,.fancy h2,.fancy h3,.fancy h4 {font-weight:normal;color:#044a64}
.fancy h2 { margin-left: 10px }
.fancy h3 { margin-left: 20px }
.fancy h4 { margin-left: 30px }
.fancy th {white-space:nowrap;text-align:left;border-bottom:solid 1px #444}
.fancy th, .fancy td {padding: 0.2em 1ex; vertical-align:top}
.fancy #toc a { color: darkblue ; text-decoration: none }
.fancy .todo { color: #AA3333 ; font-style : italic }
.fancy .todo:before { content: 'TODO:' }
.fancy p.todo { border: solid #AA3333 1px; padding: 1ex }
.fancy img { display:block; }
.fancy :link:hover, .fancy :visited:hover { background: wheat }
.fancy p,.fancy ul,.fancy ol { margin: 1em 5ex }
.fancy li p { margin: 1em 0 }
/* End of "fancyformat" specific rules. */
</style>
</head>
<body>
<div><!-- container div to satisfy validator -->
<a href="index.html">
<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite Logo"
border="0"></a>
<div><!-- IE hack to prevent disappearing logo--></div>
<div class="tagline">Small. Fast. Reliable.<br>Choose any three.</div>
<table width=100% class="menubar"><tr>
<td width=100%>
<div class="toolbar">
<a href="about.html">About</a>
<a href="sitemap.html">Sitemap</a>
<a href="docs.html">Documentation</a>
<a href="download.html">Download</a>
<a href="copyright.html">License</a>
<a href="news.html">News</a>
<a href="support.html">Support</a>
</div>
<script>
gMsg = "Search SQLite Docs..."
function entersearch() {
var q = document.getElementById("q");
if( q.value == gMsg ) { q.value = "" }
q.style.color = "black"
q.style.fontStyle = "normal"
}
function leavesearch() {
var q = document.getElementById("q");
if( q.value == "" ) {
q.value = gMsg
q.style.color = "#044a64"
q.style.fontStyle = "italic"
}
}
function hideorshow(btn,obj){
var x = document.getElementById(obj);
var b = document.getElementById(btn);
if( x.style.display!='none' ){
x.style.display = 'none';
b.innerHTML='show';
}else{
x.style.display = '';
b.innerHTML='hide';
}
return false;
}
</script>
<td>
<div style="padding:0 1em 0px 0;white-space:nowrap">
<form name=f method="GET" action="http://www.sqlite.org/search">
<input id=q name=q type=text
onfocus="entersearch()" onblur="leavesearch()" style="width:24ex;padding:1px 1ex; border:solid white 1px; font-size:0.9em ; font-style:italic;color:#044a64;" value="Search SQLite Docs...">
<input type=submit value="Go" style="border:solid white 1px;background-color:#044a64;color:white;font-size:0.9em;padding:0 1ex">
</form>
</div>
</table>
<div class=startsearch></div>
<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.
An 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 outweighed 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 treat 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>
|
