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257 lines
8.1 KiB
XML
257 lines
8.1 KiB
XML
<?xml version='1.0' encoding='iso-8859-1'?>
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<!-- $Revision: 1.8 $ -->
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<refentry xml:id="function.PDO-sqliteCreateAggregate" xmlns="http://docbook.org/ns/docbook">
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<refnamediv>
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<refname>PDO->sqliteCreateAggregate()</refname>
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<refpurpose>
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Registers an aggregating User Defined Function for use in SQL statements
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</refpurpose>
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</refnamediv>
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<refsect1 role="description">
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&reftitle.description;
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<classsynopsis>
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<ooclass><classname>PDO</classname></ooclass>
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<methodsynopsis>
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<type>bool</type><methodname>sqliteCreateAggregate</methodname>
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<methodparam><type>string</type><parameter>function_name</parameter></methodparam>
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<methodparam><type>callback</type><parameter>step_func</parameter></methodparam>
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<methodparam><type>callback</type><parameter>finalize_func</parameter></methodparam>
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<methodparam choice="opt"><type>int</type><parameter>num_args</parameter></methodparam>
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</methodsynopsis>
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</classsynopsis>
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&warn.experimental.func;
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<para>
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This method is similar to <xref linkend="function.PDO-sqliteCreateFunction"
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/> except that it registers functions that can be used to calculate a
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result aggregated across all the rows of a query.
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</para>
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<para>
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The key difference between this method and <xref
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linkend="function.PDO-sqliteCreateFunction" /> is that two functions are
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required to manage the aggregate.
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</para>
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</refsect1>
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<refsect1 role="parameters">
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&reftitle.parameters;
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<para>
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<variablelist>
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<varlistentry>
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<term><parameter>function_name</parameter></term>
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<listitem>
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<para>
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The name of the function used in SQL statements.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>step_func</parameter></term>
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<listitem>
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<para>
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Callback function called for each row of the result set. Your PHP
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function should accumulate the result and store it in the aggregation
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context.
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</para>
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<para>
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This function need to be defined as:
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<methodsynopsis>
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<methodname><replaceable>step</replaceable></methodname>
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<methodparam><type>mixed</type><parameter>context</parameter></methodparam>
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<methodparam><type>int</type><parameter>rownumber</parameter></methodparam>
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<methodparam><type>mixed</type><parameter>value1</parameter></methodparam>
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<methodparam choice="opt"><type>mixed</type><parameter>value2</parameter></methodparam>
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<methodparam choice="opt"><type>mixed</type><parameter>..</parameter></methodparam>
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</methodsynopsis>
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</para>
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<para>
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<varname>context</varname> will be &null; for the first row; on
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subsequent rows it will have the value that was previously returned
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from the step function; you should use this to maintain the aggregate
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state.
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</para>
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<para>
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<varname>rownumber</varname> will hold the current row number.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>finalize_func</parameter></term>
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<listitem>
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<para>
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Callback function to aggregate the "stepped" data from each row.
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Once all the rows have been processed, this function will be called
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and it should then take the data from the aggregation context and
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return the result. Callback functions should return a type understood
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by SQLite (i.e. <link
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linkend="language.types.intro">scalar type</link>).
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</para>
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<para>
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This function need to be defined as:
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<methodsynopsis>
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<methodname><replaceable>fini</replaceable></methodname>
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<methodparam><type>mixed</type><parameter>context</parameter></methodparam>
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<methodparam><type>int</type><parameter>rownumber</parameter></methodparam>
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</methodsynopsis>
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</para>
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<para>
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<varname>context</varname> will hold the return value from the very
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last call to the step function.
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</para>
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<para>
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<varname>rownumber</varname> will hold the number of rows over which
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the aggregate was performed.
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</para>
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<para>
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The return value of this function will be used as the return value for
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the aggregate.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>num_args</parameter></term>
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<listitem>
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<para>
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Hint to the SQLite parser if the callback function accepts a
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predetermined number of arguments.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect1>
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<refsect1 role="returnvalues">
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&reftitle.returnvalues;
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<para>
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&return.success;
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</para>
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</refsect1>
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<refsect1 role="examples">
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&reftitle.examples;
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<para>
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<example>
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<title>max_length aggregation function example</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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$data = array(
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'one',
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'two',
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'three',
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'four',
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'five',
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'six',
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'seven',
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'eight',
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'nine',
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'ten',
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);
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$db = new PDO('sqlite::memory:');
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$db->exec("CREATE TABLE strings(a)");
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$insert = $db->prepare('INSERT INTO strings VALUES (?)');
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foreach ($data as $str) {
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$insert->execute(array($str));
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}
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$insert = null;
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function max_len_step(&$context, $rownumber, $string)
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{
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if (strlen($string) > $context) {
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$context = strlen($string);
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}
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}
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function max_len_finalize(&$context, $rownumber)
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{
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return $context;
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}
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$db->sqliteCreateAggregate('max_len', 'max_len_step', 'max_len_finalize');
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var_dump($db->query('SELECT max_len(a) from strings')->fetchAll());
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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In this example, we are creating an aggregating function that will
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calculate the length of the longest string in one of the columns of the
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table. For each row, the <literal>max_len_step</literal> function is
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called and passed a <parameter>context</parameter> parameter. The context
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parameter is just like any other PHP variable and be set to hold an array
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or even an object value. In this example, we are simply using it to hold
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the maximum length we have seen so far; if the
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<parameter>string</parameter> has a length longer than the current
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maximum, we update the context to hold this new maximum length.
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</para>
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<para>
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After all of the rows have been processed, SQLite calls the
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<literal>max_len_finalize</literal> function to determine the aggregate
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result. Here, we could perform some kind of calculation based on the
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data found in the <parameter>context</parameter>. In our simple example
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though, we have been calculating the result as the query progressed, so we
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simply need to return the context value.
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</para>
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<tip>
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<para>
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It is NOT recommended for you to store a copy of the values in the context
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and then process them at the end, as you would cause SQLite to use a lot of
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memory to process the query - just think of how much memory you would need
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if a million rows were stored in memory, each containing a string 32 bytes
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in length.
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</para>
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</tip>
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<tip>
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<para>
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You can use <xref linkend="function.PDO-sqliteCreateFunction" /> and
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<xref linkend="function.PDO-sqliteCreateAggregate" /> to override SQLite
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native SQL functions.
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</para>
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</tip>
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<note>
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<para>
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This method is not available with the SQLite2 driver.
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Use the old style sqlite API for that instead.
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</para>
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</note>
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</refsect1>
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<refsect1 role="seealso">
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&reftitle.seealso;
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<para>
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<simplelist>
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<member><xref linkend="function.PDO-sqliteCreateFunction" /></member>
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<member><function>sqlite_create_function</function></member>
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<member><function>sqlite_create_aggregate</function></member>
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</simplelist>
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</para>
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</refsect1>
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</refentry>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"../../../../manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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