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NUL Characters In Strings
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<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 to and excluding
the first NUL.
</p></li><li><p>
The <a href="lang_corefunc.html#quote">quote() SQL function</a> only shows characters up to 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
of 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>
<p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/nulinstr.in?m=b99fab59ede12dcc2">2022-05-23 22:21:54</a> UTC </small></i></p>
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