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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=b99fab59ed">2022-05-23 22:21:54</a> UTC </small></i></p> + |