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<title>Determine If An SQL Statement Writes The Database</title>
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<div class=nosearch>
<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
<h2>Determine If An SQL Statement Writes The Database</h2>
</div>
<blockquote><pre>
int sqlite3_stmt_readonly(sqlite3_stmt *pStmt);
</pre></blockquote>
<p>
The sqlite3_stmt_readonly(X) interface returns true (non-zero) if
and only if the <a href="../c3ref/stmt.html">prepared statement</a> X makes no direct changes to
the content of the database file.</p>
<p>Note that <a href="../appfunc.html">application-defined SQL functions</a> or
<a href="../vtab.html">virtual tables</a> might change the database indirectly as a side effect.
For example, if an application defines a function "eval()" that
calls <a href="../c3ref/exec.html">sqlite3_exec()</a>, then the following SQL statement would
change the database file through side-effects:</p>
<p><blockquote><pre>
SELECT eval('DELETE FROM t1') FROM t2;
</pre></blockquote></p>
<p>But because the <a href="../lang_select.html">SELECT</a> statement does not change the database file
directly, sqlite3_stmt_readonly() would still return true.</p>
<p>Transaction control statements such as <a href="../lang_transaction.html">BEGIN</a>, <a href="../lang_transaction.html">COMMIT</a>, <a href="../lang_transaction.html">ROLLBACK</a>,
<a href="../lang_savepoint.html">SAVEPOINT</a>, and <a href="../lang_savepoint.html">RELEASE</a> cause sqlite3_stmt_readonly() to return true,
since the statements themselves do not actually modify the database but
rather they control the timing of when other statements modify the
database. The <a href="../lang_attach.html">ATTACH</a> and <a href="../lang_detach.html">DETACH</a> statements also cause
sqlite3_stmt_readonly() to return true since, while those statements
change the configuration of a database connection, they do not make
changes to the content of the database files on disk.
The sqlite3_stmt_readonly() interface returns true for <a href="../lang_transaction.html">BEGIN</a> since
<a href="../lang_transaction.html">BEGIN</a> merely sets internal flags, but the <a href="../lang_transaction.html">BEGIN IMMEDIATE</a> and
<a href="../lang_transaction.html">BEGIN EXCLUSIVE</a> commands do touch the database and so
sqlite3_stmt_readonly() returns false for those commands.</p>
<p>This routine returns false if there is any possibility that the
statement might change the database file. A false return does
not guarantee that the statement will change the database file.
For example, an UPDATE statement might have a WHERE clause that
makes it a no-op, but the sqlite3_stmt_readonly() result would still
be false. Similarly, a CREATE TABLE IF NOT EXISTS statement is a
read-only no-op if the table already exists, but
sqlite3_stmt_readonly() still returns false for such a statement.</p>
<p>If prepared statement X is an <a href="../lang_explain.html">EXPLAIN</a> or <a href="../eqp.html">EXPLAIN QUERY PLAN</a>
statement, then sqlite3_stmt_readonly(X) returns the same value as
if the EXPLAIN or EXPLAIN QUERY PLAN prefix were omitted.
</p><p>See also lists of
<a href="../c3ref/objlist.html">Objects</a>,
<a href="../c3ref/constlist.html">Constants</a>, and
<a href="../c3ref/funclist.html">Functions</a>.</p>
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