Php/docs/sqlite3.createaggregate
SQLite3::createAggregate
(PHP 5 >= 5.3.0, PHP 7)
SQLite3::createAggregate — Registers a PHP function for use as an SQL aggregate function
说明
public SQLite3::createAggregate
( string $name
, mixed $step_callback
, mixed $final_callback
[, int $argument_count = -1
] ) : bool
Registers a PHP function or user-defined function for use as an SQL aggregate function for use within SQL statements.
参数
nameName of the SQL aggregate to be created or redefined.
step_callbackCallback function called for each row of the result set. Your PHP function should accumulate the result and store it in the aggregation context.
This function need to be defined as:
contextnullfor the first row; on subsequent rows it will have the value that was previously returned from the step function; you should use this to maintain the aggregate state.rownumberThe current row number.
valueThe first argument passed to the aggregate.
valuesFurther arguments passed to the aggregate.
The return value of this function will be used as the
contextargument in the next call of the step or finalize functions.final_callbackCallback function to aggregate the "stepped" data from each row. Once all the rows have been processed, this function will be called and it should then take the data from the aggregation context and return the result. This callback function should return a type understood by SQLite (i.e. scalar type).
This function need to be defined as:
contextHolds the return value from the very last call to the step function.
rownumberAlways
0.
The return value of this function will be used as the return value for the aggregate.
argument_countThe number of arguments that the SQL aggregate takes. If this parameter is negative, then the SQL aggregate may take any number of arguments.
返回值
Returns true upon successful creation of the aggregate, 或者在失败时返回 false.
范例
Example #1 max_length aggregation function example
<?php$data = array( 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine', 'ten', );$db = new SQLite3(':memory:');$db->exec("CREATE TABLE strings(a)");$insert = $db->prepare('INSERT INTO strings VALUES (?)');foreach ($data as $str) { $insert->bindValue(1, $str); $insert->execute();}$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 === null ? 0 : $context;}$db->createAggregate('max_len', 'max_len_step', 'max_len_finalize');var_dump($db->querySingle('SELECT max_len(a) from strings'));?>
以上例程会输出：
int(5)
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.
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 SQLite3::createAggregate() to override SQLite native SQL functions.
