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PDO::sqliteCreateAggregate — Registers an aggregating User Defined Function for use in SQL statements
本函数是实验性的。本函数的行为，包括函数名称以及其它任何关于本函数的文档可能会在没有通知的情况下随 PHP 以后的发布而改变。使用本函数风险自担。
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.
The name of the function used in SQL statements.
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:
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.
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:
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.
Hint to the SQLite parser if the callback function accepts a predetermined number of arguments.
如果成功则返回 TRUE，失败则返回 FALSE。
Example #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.
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.
You can use PDO::sqliteCreateFunction and PDO::sqliteCreateAggregate to override SQLite native SQL functions.
Note: This method is not available with the SQLite2 driver. Use the old style sqlite API for that instead.