PHP Manual: Register an aggregating UDF for use in SQL statements

Description

void sqlite_create_aggregate ( resource $dbhandle , string $function_name , callable $step_func , callable $finalize_func [, int $num_args = -1 ] )

Object oriented style (method):

public void SQLiteDatabase::createAggregate ( string $function_name , callable $step_func , callable $finalize_func [, int $num_args = -1 ] )

sqlite_create_aggregate is similar to sqlite_create_function except that it registers functions that can be used to calculate a result aggregated across all the rows of a query.

The key difference between this function and sqlite_create_function is that two functions are required to manage the aggregate; step_func 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, finalize_func 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. scalar type).

Parameters

dbhandle: The SQLite Database resource; returned from sqlite_open when used procedurally. This parameter is not required when using the object-oriented method.
function_name: The name of the function used in SQL statements.
step_func: Callback function called for each row of the result set. Function parameters are &$context, $value, ....
finalize_func: Callback function to aggregate the "stepped" data from each row. Function parameter is &$context and the function should return the final result of aggregation.
num_args: Hint to the SQLite parser if the callback function accepts a predetermined number of arguments.

Return Values

No value is returned.

Examples

Example #1 max_length aggregation function example


<?php
$data = array(
   'one',
   'two',
   'three',
   'four',
   'five',
   'six',
   'seven',
   'eight',
   'nine',
   'ten',
   );
$dbhandle = sqlite_open(':memory:');
sqlite_query($dbhandle, "CREATE TABLE strings(a)");
foreach ($data as $str) {
    $str = sqlite_escape_string($str);
    sqlite_query($dbhandle, "INSERT INTO strings VALUES ('$str')");
}

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

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

sqlite_create_aggregate($dbhandle, 'max_len', 'max_len_step', 'max_len_finalize');

var_dump(sqlite_array_query($dbhandle, 'SELECT max_len(a) from strings'));

?>

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 max_len_step function is called and passed a context 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 string has a length longer than the current maximum, we update the context to hold this new maximum length.

After all of the rows have been processed, SQLite calls the max_len_finalize function to determine the aggregate result. Here, we could perform some kind of calculation based on the data found in the context. In our simple example though, we have been calculating the result as the query progressed, so we simply need to return the context value.

Note:
The example above will not work correctly if the column contains binary data. Take a look at the manual page for sqlite_udf_decode_binary for an explanation of why this is so, and an example of how to make it respect the binary encoding.

Tip

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.

Tip

You can use sqlite_create_function and sqlite_create_aggregate to override SQLite native SQL functions.

sqlite_create_aggregate

SQLiteDatabase::createAggregate

Description

Parameters

Return Values

Examples

See Also