213 lines
6.4 KiB
HTML
213 lines
6.4 KiB
HTML
<!DOCTYPE html>
|
|
<html><head>
|
|
<meta name="viewport" content="width=device-width, initial-scale=1.0">
|
|
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
|
|
<link href="sqlite.css" rel="stylesheet">
|
|
<title>NUL Characters In Strings</title>
|
|
<!-- path= -->
|
|
</head>
|
|
<body>
|
|
<div class=nosearch>
|
|
<a href="index.html">
|
|
<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0">
|
|
</a>
|
|
<div><!-- IE hack to prevent disappearing logo --></div>
|
|
<div class="tagline desktoponly">
|
|
Small. Fast. Reliable.<br>Choose any three.
|
|
</div>
|
|
<div class="menu mainmenu">
|
|
<ul>
|
|
<li><a href="index.html">Home</a>
|
|
<li class='mobileonly'><a href="javascript:void(0)" onclick='toggle_div("submenu")'>Menu</a>
|
|
<li class='wideonly'><a href='about.html'>About</a>
|
|
<li class='desktoponly'><a href="docs.html">Documentation</a>
|
|
<li class='desktoponly'><a href="download.html">Download</a>
|
|
<li class='wideonly'><a href='copyright.html'>License</a>
|
|
<li class='desktoponly'><a href="support.html">Support</a>
|
|
<li class='desktoponly'><a href="prosupport.html">Purchase</a>
|
|
<li class='search' id='search_menubutton'>
|
|
<a href="javascript:void(0)" onclick='toggle_search()'>Search</a>
|
|
</ul>
|
|
</div>
|
|
<div class="menu submenu" id="submenu">
|
|
<ul>
|
|
<li><a href='about.html'>About</a>
|
|
<li><a href='docs.html'>Documentation</a>
|
|
<li><a href='download.html'>Download</a>
|
|
<li><a href='support.html'>Support</a>
|
|
<li><a href='prosupport.html'>Purchase</a>
|
|
</ul>
|
|
</div>
|
|
<div class="searchmenu" id="searchmenu">
|
|
<form method="GET" action="search">
|
|
<select name="s" id="searchtype">
|
|
<option value="d">Search Documentation</option>
|
|
<option value="c">Search Changelog</option>
|
|
</select>
|
|
<input type="text" name="q" id="searchbox" value="">
|
|
<input type="submit" value="Go">
|
|
</form>
|
|
</div>
|
|
</div>
|
|
<script>
|
|
function toggle_div(nm) {
|
|
var w = document.getElementById(nm);
|
|
if( w.style.display=="block" ){
|
|
w.style.display = "none";
|
|
}else{
|
|
w.style.display = "block";
|
|
}
|
|
}
|
|
function toggle_search() {
|
|
var w = document.getElementById("searchmenu");
|
|
if( w.style.display=="block" ){
|
|
w.style.display = "none";
|
|
} else {
|
|
w.style.display = "block";
|
|
setTimeout(function(){
|
|
document.getElementById("searchbox").focus()
|
|
}, 30);
|
|
}
|
|
}
|
|
function div_off(nm){document.getElementById(nm).style.display="none";}
|
|
window.onbeforeunload = function(e){div_off("submenu");}
|
|
/* Disable the Search feature if we are not operating from CGI, since */
|
|
/* Search is accomplished using CGI and will not work without it. */
|
|
if( !location.origin || !location.origin.match || !location.origin.match(/http/) ){
|
|
document.getElementById("search_menubutton").style.display = "none";
|
|
}
|
|
/* Used by the Hide/Show button beside syntax diagrams, to toggle the */
|
|
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>
|
|
</div>
|
|
<div class=fancy>
|
|
<div class=nosearch>
|
|
<div class="fancy_title">
|
|
NUL Characters In Strings
|
|
</div>
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<h1 id="introduction"><span>1. </span>Introduction</h1>
|
|
|
|
<p>
|
|
SQLite allows NUL characters (ASCII 0x00, Unicode \u0000) in the middle
|
|
of string values stored in the database. However, the use of NUL within
|
|
strings can lead to surprising behaviors:
|
|
|
|
</p><ol>
|
|
<li><p>
|
|
The <a href="lang_corefunc.html#length">length() SQL function</a> only counts characters up through and excluding
|
|
the first NUL.
|
|
|
|
|
|
</p></li><li><p>
|
|
The <a href="lang_corefunc.html#quote">quote() SQL function</a> only shows characters up through and excluding
|
|
the first NUL.
|
|
|
|
</p></li><li><p>
|
|
The <a href="cli.html#dump">.dump</a> command in the <a href="cli.html">CLI</a> omits the first NUL character and all
|
|
subsequent text in the SQL output that it generates. In fact, the
|
|
<a href="cli.html">CLI</a> omits everything past the first NUL character in all contexts.
|
|
</p></li></ol>
|
|
|
|
<p>
|
|
The use of NUL characters in SQL text strings is not recommended.
|
|
|
|
</p><h1 id="unexpected_behavior"><span>2. </span>Unexpected Behavior</h1>
|
|
|
|
<p>
|
|
Consider the following SQL:
|
|
|
|
</p><div class="codeblock"><pre>CREATE TABLE t1(
|
|
a INTEGER PRIMARY KEY,
|
|
b TEXT
|
|
);
|
|
INSERT INTO t1(a,b) VALUES(1, 'abc'||char(0)||'xyz');
|
|
|
|
SELECT a, b, length(b) FROM t1;
|
|
</pre></div>
|
|
|
|
<p>
|
|
The SELECT statement above shows output of:
|
|
|
|
</p><div class="codeblock"><pre>1,'abc',3
|
|
</pre></div>
|
|
|
|
<p>
|
|
(Through this document, we assume that the <a href="cli.html">CLI</a> has "<a href="cli.html#dotmodequote">.mode quote</a>" set.)
|
|
But if you run:
|
|
|
|
</p><div class="codeblock"><pre>SELECT * FROM t1 WHERE b='abc';
|
|
</pre></div>
|
|
|
|
<p>
|
|
Then no rows are returned. SQLite knows that the t1.b column actually
|
|
holds a 7-character string, and the 7-character string 'abc'||char(0)||'xyz'
|
|
is not equal to the 3-character string 'abc', and so no rows are returned.
|
|
But a user might be easily confused by this because the <a href="cli.html">CLI</a> output
|
|
seems to show that the string has only 3 characters. This seems like
|
|
a bug. But it is how SQLite works.
|
|
|
|
</p><h1 id="how_to_tell_if_you_have_nul_characters_in_your_strings"><span>3. </span>How To Tell If You Have NUL Characters In Your Strings</h1>
|
|
|
|
<p>
|
|
If you <a href="lang_expr.html#castexpr">CAST</a> a string into a BLOB, then the entire length of the
|
|
string is shown. For example:
|
|
|
|
</p><div class="codeblock"><pre>SELECT a, CAST(b AS BLOB) FROM t1;
|
|
</pre></div>
|
|
|
|
<p>
|
|
Gives this result:
|
|
|
|
</p><div class="codeblock"><pre>1,X'6162630078797a'
|
|
</pre></div>
|
|
|
|
<p>
|
|
In the BLOB output, you can clearly see the NUL character as the 4th
|
|
character in the 7-character string.
|
|
|
|
</p><p>
|
|
Another, more automated, way
|
|
to tell if a string value X contains embedded NUL characters is to
|
|
use an expression like this:
|
|
|
|
</p><div class="codeblock"><pre>instr(X,char(0))
|
|
</pre></div>
|
|
|
|
<p>
|
|
If this expression returns a non-zero value N, then there exists an
|
|
embedded NUL at the N-th character position. Thus to count the number
|
|
fo rows that contain embedded NUL characters:
|
|
|
|
</p><div class="codeblock"><pre>SELECT count(*) FROM t1 WHERE instr(b,char(0))>0;
|
|
</pre></div>
|
|
|
|
<h1 id="removing_nul_characters_from_a_text_field"><span>4. </span>Removing NUL Characters From A Text Field</h1>
|
|
|
|
<p>
|
|
The following example shows how to remove NUL character, and all text
|
|
that follows, from a column of a table. So if you have a database file
|
|
that contains embedded NULs and you would like to remove them, running
|
|
UPDATE statements similar to the following might help:
|
|
|
|
</p><div class="codeblock"><pre>UPDATE t1 SET b=substr(b,1,instr(b,char(0)))
|
|
WHERE instr(b,char(0));
|
|
</pre></div>
|
|
|