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.
PDO::sqliteCreateAggregate() 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 function and
PDO::sqliteCreateFunction() 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).
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.
finalize_func
Callback function to aggregate the "stepped" data from each row.
num_args
Hint to the SQLite parser if the callback function accepts a
predetermined number of arguments.
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.