Registers an aggregating User Defined Function for use in SQL statements


(PHP 5 >= 5.1.0, PECL pdo_sqlite >= 1.0.0)

PDO::sqliteCreateAggregate Registers an aggregating User Defined Function for use in SQL statements

Описание

public bool PDO::sqliteCreateAggregate ( string $function_name , callable $step_func , callable $finalize_func [, int $num_args ] )
Внимание

Эта функция является ЭКСПЕРИМЕНТАЛЬНОЙ. Поведение этой функции, ее имя и относящаяся к ней документация могут измениться в последующих версиях PHP без уведомления. Используйте эту функцию на свой страх и риск.

This method is similar to PDO::sqliteCreateFunction 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 method and PDO::sqliteCreateFunction is that two functions are required to manage the aggregate.

Список параметров

function_name

The name of the function used in SQL statements.

step_func

Callback function called for each row of the result set. Your PHP function should accumulate the result and store it in the aggregation context.

This function need to be defined as:

mixed step ( mixed $context , int $rownumber , mixed $value1 [, mixed $value2 [, mixed $.. ]] )

context 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.

rownumber will hold the current row number.

The return value of this function will be used as the context argument in the next call of the step or finalize functions.

finalize_func

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. scalar type).

This function need to be defined as:

mixed fini ( mixed $context , int $rownumber )

context will hold the return value from the very last call to the step function.

rownumber will hold the number of rows over which the aggregate was performed.

The return value of this function will be used as the return value for the aggregate.

num_args

Hint to the SQLite parser if the callback function accepts a predetermined number of arguments.

Возвращаемые значения

Возвращает TRUE в случае успешного завершения или FALSE в случае возникновения ошибки.

Примеры

Пример #1 max_length aggregation function example

<?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());

?>

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.

Подсказка

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.

Подсказка

You can use PDO::sqliteCreateFunction and PDO::sqliteCreateAggregate to override SQLite native SQL functions.

Замечание:

This method is not available with the SQLite2 driver. Use the old style sqlite API for that instead.

Смотрите также