Source of: /manual/en/function.sqlite-create-aggregate.php
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<div id="function.sqlite-create-aggregate" class="refentry">
<div class="refnamediv">
<h1 class="refname">sqlite_create_aggregate</h1>
<h1 class="refname">SQLiteDatabase->createAggregate</h1>
<p class="verinfo">(PHP 5, PECL sqlite >= 1.0.0)</p><p class="refpurpose"><span class="refname">sqlite_create_aggregate</span> -- <span class="refname">SQLiteDatabase->createAggregate</span> — <span class="dc-title">Register an aggregating UDF for use in SQL statements</span></p>
</div>
<a name="function.sqlite-create-aggregate.description"></a><div class="refsect1 description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type"><span class="type void">void</span></span> <span class="methodname"><b>sqlite_create_aggregate</b></span>
( <span class="methodparam"><span class="type">resource</span> <tt class="parameter">$dbhandle</tt></span>
, <span class="methodparam"><span class="type">string</span> <tt class="parameter">$function_name</tt></span>
, <span class="methodparam"><span class="type"><a href="language.pseudo-types.php#language.types.callback" class="type callback">callback</a></span> <tt class="parameter">$step_func</tt></span>
, <span class="methodparam"><span class="type"><a href="language.pseudo-types.php#language.types.callback" class="type callback">callback</a></span> <tt class="parameter">$finalize_func</tt></span>
[, <span class="methodparam"><span class="type">int</span> <tt class="parameter">$num_args</tt><span class="initializer"> = -1</span></span>
] )</div>
<p class="para rdfs-comment">Object oriented style (method):</p>
<div class="classsynopsis">
<div class="ooclass"><b class="classname">SQLiteDatabase</b></div>
<div class="methodsynopsis dc-description">
<span class="type">void</span> <span class="methodname"><b>createAggregate</b></span>
( <span class="methodparam"><span class="type">string</span> <tt class="parameter">$function_name</tt></span>
, <span class="methodparam"><span class="type"><a href="language.pseudo-types.php#language.types.callback" class="type callback">callback</a></span> <tt class="parameter">$step_func</tt></span>
, <span class="methodparam"><span class="type"><a href="language.pseudo-types.php#language.types.callback" class="type callback">callback</a></span> <tt class="parameter">$finalize_func</tt></span>
[, <span class="methodparam"><span class="type">int</span> <tt class="parameter">$num_args</tt><span class="initializer"> = -1</span></span>
] )</div>
</div>
<p class="para">
<b>sqlite_create_aggregate()</b> is similar to
<a href="function.sqlite-create-function.php" class="function">sqlite_create_function()</a> except that it registers
functions that can be used to calculate a result aggregated across all the
rows of a query.
</p>
<p class="para">
The key difference between this function and
<a href="function.sqlite-create-function.php" class="function">sqlite_create_function()</a> is that two functions are
required to manage the aggregate; <i><tt class="parameter">step_func</tt></i>
is
called for each row of the result set. Your PHP function should
accumulate the result and store it into the aggregation context.
Once all the rows have been processed,
<i><tt class="parameter">finalize_func</tt></i>
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.
<a href="language.types.intro.php" class="link">scalar type</a>).
</p>
</div>
<a name="function.sqlite-create-aggregate.parameters"></a><div class="refsect1 parameters">
<h3 class="title">Parameters</h3>
<p class="para">
</p><dl>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">dbhandle</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
The SQLite Database resource; returned from <a href="function.sqlite-open.php" class="function">sqlite_open()</a>
when used procedurally. This parameter is not required
when using the object-oriented method.
</p>
</dd>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">function_name</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
The name of the function used in SQL statements.
</p>
</dd>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">step_func</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
Callback function called for each row of the result set.
</p>
</dd>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">finalize_func</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
Callback function to aggregate the "stepped" data from each row.
</p>
</dd>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">num_args</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
Hint to the SQLite parser if the callback function accepts a
predetermined number of arguments.
</p>
</dd>
</dl>
<p>
</p>
</div>
<a name="function.sqlite-create-aggregate.returnvalues"></a><div class="refsect1 returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
No value is returned.
</p>
</div>
<a name="function.sqlite-create-aggregate.examples"></a><div class="refsect1 examples">
<h3 class="title">Examples</h3>
<p class="para">
</p><div class="example">
<p><b>Example #1 max_length aggregation function example</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$data </span><span style="color: #007700">= array(<br /> </span><span style="color: #DD0000">'one'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'two'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'three'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'four'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'five'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'six'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'seven'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'eight'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'nine'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'ten'</span><span style="color: #007700">,<br /> );<br /></span><span style="color: #0000BB">$dbhandle </span><span style="color: #007700">= </span><span style="color: #0000BB">sqlite_open</span><span style="color: #007700">(</span><span style="color: #DD0000">':memory:'</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">sqlite_query</span><span style="color: #007700">(</span><span style="color: #0000BB">$dbhandle</span><span style="color: #007700">, </span><span style="color: #DD0000">"CREATE TABLE strings(a)"</span><span style="color: #007700">);<br />foreach (</span><span style="color: #0000BB">$data </span><span style="color: #007700">as </span><span style="color: #0000BB">$str</span><span style="color: #007700">) {<br /> </span><span style="color: #0000BB">$str </span><span style="color: #007700">= </span><span style="color: #0000BB">sqlite_escape_string</span><span style="color: #007700">(</span><span style="color: #0000BB">$str</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">sqlite_query</span><span style="color: #007700">(</span><span style="color: #0000BB">$dbhandle</span><span style="color: #007700">, </span><span style="color: #DD0000">"INSERT INTO strings VALUES ('</span><span style="color: #0000BB">$str</span><span style="color: #DD0000">')"</span><span style="color: #007700">);<br />}<br /><br />function </span><span style="color: #0000BB">max_len_step</span><span style="color: #007700">(&</span><span style="color: #0000BB">$context</span><span style="color: #007700">, </span><span style="color: #0000BB">$string</span><span style="color: #007700">) <br />{<br /> if (</span><span style="color: #0000BB">strlen</span><span style="color: #007700">(</span><span style="color: #0000BB">$string</span><span style="color: #007700">) > </span><span style="color: #0000BB">$context</span><span style="color: #007700">) {<br /> </span><span style="color: #0000BB">$context </span><span style="color: #007700">= </span><span style="color: #0000BB">strlen</span><span style="color: #007700">(</span><span style="color: #0000BB">$string</span><span style="color: #007700">);<br /> }<br />}<br /><br />function </span><span style="color: #0000BB">max_len_finalize</span><span style="color: #007700">(&</span><span style="color: #0000BB">$context</span><span style="color: #007700">) <br />{<br /> return </span><span style="color: #0000BB">$context</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">sqlite_create_aggregate</span><span style="color: #007700">(</span><span style="color: #0000BB">$dbhandle</span><span style="color: #007700">, </span><span style="color: #DD0000">'max_len'</span><span style="color: #007700">, </span><span style="color: #DD0000">'max_len_step'</span><span style="color: #007700">, </span><span style="color: #DD0000">'max_len_finalize'</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">sqlite_array_query</span><span style="color: #007700">(</span><span style="color: #0000BB">$dbhandle</span><span style="color: #007700">, </span><span style="color: #DD0000">'SELECT max_len(a) from strings'</span><span style="color: #007700">));<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div><p>
</p>
<p class="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 <i>max_len_step</i> function is
called and passed a <i><tt class="parameter">context</tt></i>
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
<i><tt class="parameter">string</tt></i>
has a length longer than the current
maximum, we update the context to hold this new maximum length.
</p>
<p class="para">
After all of the rows have been processed, SQLite calls the
<i>max_len_finalize</i> function to determine the aggregate
result. Here, we could perform some kind of calculation based on the
data found in the <i><tt class="parameter">context</tt></i>
. In our simple example
though, we have been calculating the result as the query progressed, so we
simply need to return the context value.
</p>
<blockquote><p><b class="note">Note</b>:
The example above will not work correctly if the column contains binary
data. Take a look at the manual page for
<a href="function.sqlite-udf-decode-binary.php" class="function">sqlite_udf_decode_binary()</a> for an explanation of why
this is so, and an example of how to make it respect the binary encoding.
<br />
</p></blockquote>
<div class="tip"><b class="tip">Tip</b>
<p class="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.
</p>
</div>
<div class="tip"><b class="tip">Tip</b>
<p class="para">
You can use <a href="function.sqlite-create-function.php" class="function">sqlite_create_function()</a> and
<b>sqlite_create_aggregate()</b> to override SQLite native
SQL functions.
</p>
</div>
</div>
<a name="function.sqlite-create-aggregate.seealso"></a><div class="refsect1 seealso">
<h3 class="title">See Also</h3>
<p class="para">
</p><ul class="simplelist">
<li class="member"><a href="function.sqlite-create-function.php" class="function" rel="rdfs-seeAlso">sqlite_create_function()</a> - Registers a "regular" User Defined Function for use in SQL statements</li>
<li class="member"><a href="function.sqlite-udf-encode-binary.php" class="function" rel="rdfs-seeAlso">sqlite_udf_encode_binary()</a> - Encode binary data before returning it from an UDF</li>
<li class="member"><a href="function.sqlite-udf-decode-binary.php" class="function" rel="rdfs-seeAlso">sqlite_udf_decode_binary()</a> - Decode binary data passed as parameters to an UDF</li>
</ul><p>
</p>
</div>
</div><?php manual_footer(); ?>
