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<refentry xml:id="function.pdo-sqlitecreateaggregate" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>PDO::sqliteCreateAggregate</refname>
  <refpurpose>
   Registers an aggregating User Defined Function for use in SQL statements
  </refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <classsynopsis>
   <ooclass><classname>PDO</classname></ooclass>
   <methodsynopsis>
    <type>bool</type><methodname>sqliteCreateAggregate</methodname>
    <methodparam><type>string</type><parameter>function_name</parameter></methodparam>
    <methodparam><type>callback</type><parameter>step_func</parameter></methodparam>
    <methodparam><type>callback</type><parameter>finalize_func</parameter></methodparam>
    <methodparam choice="opt"><type>int</type><parameter>num_args</parameter></methodparam>
   </methodsynopsis>
  </classsynopsis>
  &warn.experimental.func;
  <para>
   This method is similar to <xref linkend="function.pdo-sqlitecreatefunction"
   /> except that it registers functions that can be used to calculate a
   result aggregated across all the rows of a query.
  </para>
  <para>
   The key difference between this method and <xref
   linkend="function.pdo-sqlitecreatefunction" /> is that two functions are
   required to manage the aggregate. 
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>function_name</parameter></term>
     <listitem>
      <para>
       The name of the function used in SQL statements.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>step_func</parameter></term>
     <listitem>
      <para>
       Callback function called for each row of the result set. Your PHP
       function should accumulate the result and store it in the aggregation
       context.
      </para>
      <para>
       This function need to be defined as:
       <methodsynopsis>
        <methodname><replaceable>step</replaceable></methodname>
        <methodparam><type>mixed</type><parameter>context</parameter></methodparam>
        <methodparam><type>int</type><parameter>rownumber</parameter></methodparam>
        <methodparam><type>mixed</type><parameter>value1</parameter></methodparam>
        <methodparam choice="opt"><type>mixed</type><parameter>value2</parameter></methodparam>
        <methodparam choice="opt"><type>mixed</type><parameter>..</parameter></methodparam>
       </methodsynopsis>
      </para>
      <para>
       <varname>context</varname> will be &null; for the first row; on
       subsequent rows it will have the value that was previously returned
       from the step function; you should use this to maintain the aggregate
       state.
      </para>
      <para>
       <varname>rownumber</varname> will hold the current row number.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>finalize_func</parameter></term>
     <listitem>
      <para>
       Callback function to aggregate the "stepped" data from each row. 
       Once all the rows have been processed, this function will be called
       and it should then take the data from the aggregation context and
       return the result. Callback functions should return a type understood
       by SQLite (i.e. <link 
       linkend="language.types.intro">scalar type</link>).
      </para>
      <para>
       This function need to be defined as:
       <methodsynopsis>
        <methodname><replaceable>fini</replaceable></methodname>
        <methodparam><type>mixed</type><parameter>context</parameter></methodparam>
        <methodparam><type>int</type><parameter>rownumber</parameter></methodparam>
       </methodsynopsis>
      </para>
      <para>
       <varname>context</varname> will hold the return value from the very
       last call to the step function.
      </para>
      <para>
       <varname>rownumber</varname> will hold the number of rows over which
       the aggregate was performed.
      </para>
      <para>
       The return value of this function will be used as the return value for
       the aggregate.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>num_args</parameter></term>
     <listitem>
      <para>
       Hint to the SQLite parser if the callback function accepts a
       predetermined number of arguments.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.success;
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title>max_length aggregation function example</title>
    <programlisting role="php">
<![CDATA[
<?php
$data = array(
   'one',
   'two',
   'three',
   'four',
   'five',
   'six',
   'seven',
   'eight',
   'nine',
   'ten',
   );
$db = new PDO('sqlite::memory:');
$db->exec("CREATE TABLE strings(a)");
$insert = $db->prepare('INSERT INTO strings VALUES (?)');
foreach ($data as $str) {
    $insert->execute(array($str));
}
$insert = null;

function max_len_step(&$context, $rownumber, $string) 
{
    if (strlen($string) > $context) {
        $context = strlen($string);
    }
	return $context;
}

function max_len_finalize(&$context, $rownumber) 
{
    return $context;
}

$db->sqliteCreateAggregate('max_len', 'max_len_step', 'max_len_finalize');

var_dump($db->query('SELECT max_len(a) from strings')->fetchAll());

?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   In this example, we are creating an aggregating function that will
   calculate the length of the longest string in one of the columns of the
   table.  For each row, the <literal>max_len_step</literal> function is
   called and passed a <parameter>context</parameter> parameter.  The context
   parameter is just like any other PHP variable and be set to hold an array
   or even an object value.  In this example, we are simply using it to hold
   the maximum length we have seen so far; if the
   <parameter>string</parameter> has a length longer than the current
   maximum, we update the context to hold this new maximum length.
  </para>
  <para>
   After all of the rows have been processed, SQLite calls the
   <literal>max_len_finalize</literal> function to determine the aggregate
   result.  Here, we could perform some kind of calculation based on the
   data found in the <parameter>context</parameter>.  In our simple example
   though, we have been calculating the result as the query progressed, so we
   simply need to return the context value.
  </para>
  <tip>
   <para>
    It is NOT recommended for you to store a copy of the values in the context
    and then process them at the end, as you would cause SQLite to use a lot of
    memory to process the query - just think of how much memory you would need
    if a million rows were stored in memory, each containing a string 32 bytes
    in length.
   </para>
  </tip>
  <tip>
   <para>
    You can use <xref linkend="function.pdo-sqlitecreatefunction" /> and
    <xref linkend="function.pdo-sqlitecreateaggregate" /> to override SQLite
    native SQL functions.
   </para>
  </tip>
  <note>
   <para>
    This method is not available with the SQLite2 driver.
    Use the old style sqlite API for that instead.
   </para>
  </note>

 </refsect1>


 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><xref linkend="function.pdo-sqlitecreatefunction" /></member>
    <member><function>sqlite_create_function</function></member>
    <member><function>sqlite_create_aggregate</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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