Standard Deviation for SQLite

Question

I\'ve searched the SQLite docs and couldn\'t find anything, but I\'ve also searched on Google and a few results appeared.

Does SQLite have any built-in Standard Devi

Answer 1

I implemented the Welford's method (the same as extension-functions.c) as a SQLite UDF:

$db->sqliteCreateAggregate('stdev',
    function (&$context, $row, $data) // step callback
    {
        if (isset($context) !== true) // $context is null at first
        {
            $context = array
            (
                'k' => 0,
                'm' => 0,
                's' => 0,
            );
        }

        if (isset($data) === true) // the standard is non-NULL values only
        {
            $context['s'] += ($data - $context['m']) * ($data - ($context['m'] += ($data - $context['m']) / ++$context['k']));
        }

        return $context;
    },
    function (&$context, $row) // fini callback
    {
        if ($context['k'] > 0) // return NULL if no non-NULL values exist
        {
            return sqrt($context['s'] / $context['k']);
        }

        return null;
    },
1);

That's in PHP ($db is the PDO object) but it should be trivial to port to another language.

SQLite is soooo cool. <3