createaggregate.xml

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<refentry xml:id="sqlite3.createaggregate" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <refnamediv>
  <refname>SQLite3::createAggregate</refname>
  <refpurpose>Registers a PHP function for use as an SQL aggregate function</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis role="SQLite3">
   <modifier>public</modifier> <type>bool</type><methodname>SQLite3::createAggregate</methodname>
   <methodparam><type>string</type><parameter>name</parameter></methodparam>
   <methodparam><type>callable</type><parameter>stepCallback</parameter></methodparam>
   <methodparam><type>callable</type><parameter>finalCallback</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>argCount</parameter><initializer>-1</initializer></methodparam>
  </methodsynopsis>
  <para>
   Registers a PHP function or user-defined function for use as an SQL
   aggregate function for use within SQL statements.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>name</parameter></term>
     <listitem>
      <para>
       Name of the SQL aggregate to be created or redefined.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>stepCallback</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>
        <type>mixed</type><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>value</parameter></methodparam>
        <methodparam rep="repeat"><type>mixed</type><parameter>values</parameter></methodparam>
       </methodsynopsis>
       <variablelist>
        <varlistentry>
         <term><parameter>context</parameter></term>
         <listitem>
          <para>
           &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>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><parameter>rownumber</parameter></term>
         <listitem>
          <para>
           The current row number.
          </para>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><parameter>value</parameter></term>
         <listitem>
          <para>
           The first argument passed to the aggregate.
          </para>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><parameter>values</parameter></term>
         <listitem>
          <para>
           Further arguments passed to the aggregate.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
       The return value of this function will be used as the
       <parameter>context</parameter> argument in the next call of the step or
       finalize functions.
      </para> 
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>finalCallback</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. This callback function 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>
        <type>mixed</type><methodname><replaceable>fini</replaceable></methodname>
        <methodparam><type>mixed</type><parameter>context</parameter></methodparam>
        <methodparam><type>int</type><parameter>rownumber</parameter></methodparam>
       </methodsynopsis>
       <variablelist>
        <varlistentry>
         <term><parameter>context</parameter></term>
         <listitem>
          <para>
           Holds the return value from the very last call to the step function.
          </para>
         </listitem>
        </varlistentry>
        <varlistentry>
         <term><parameter>rownumber</parameter></term>
         <listitem>
          <para>
           Always <literal>0</literal>.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
       The return value of this function will be used as the return value for
       the aggregate.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>argCount</parameter></term>
     <listitem>
      <para>
       The number of arguments that the SQL aggregate takes. If
       this parameter is negative, then the SQL aggregate may take
       any number of arguments.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns &true; upon successful creation of the aggregate, &return.falseforfailure;.
  </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 SQLite3(':memory:');
$db->exec("CREATE TABLE strings(a)");
$insert = $db->prepare('INSERT INTO strings VALUES (?)');
foreach ($data as $str) {
    $insert->bindValue(1, $str);
    $insert->execute();
}
$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 === null ? 0 : $context;
}

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

var_dump($db->querySingle('SELECT max_len(a) from strings'));
?>
]]>
    </programlisting>
    &example.outputs;
    <screen role="php">
<![CDATA[
int(5)
]]>
    </screen>
   </example>
  </para>
  <para>
   In this example, we are creating an aggregating function that will
   calculate the length of the longest string in one of the columns of the
   table.  For each row, the <literal>max_len_step</literal> function is
   called and passed a <literal>$context</literal> 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
   <literal>$string</literal> has a length longer than the current
   maximum, we update the context to hold this new maximum length.
  </para>
  <para>
   After all of the rows have been processed, SQLite calls the
   <literal>max_len_finalize</literal> function to determine the aggregate
   result.  Here, we could perform some kind of calculation based on the
   data found in the <literal>$context</literal>.  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 <methodname>SQLite3::createAggregate</methodname> to override SQLite
    native SQL functions.
   </para>
  </tip>
 </refsect1>

</refentry>
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