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

There is still no built-in stdev function in sqlite. However, you can define (as Alix has done) a user-defined aggregator function. Here is a complete example in Python:

import sqlite3
import math

class StdevFunc:
    def __init__(self):
        self.M = 0.0
        self.S = 0.0
        self.k = 1

    def step(self, value):
        if value is None:
            return
        tM = self.M
        self.M += (value - tM) / self.k
        self.S += (value - tM) * (value - self.M)
        self.k += 1

    def finalize(self):
        if self.k < 3:
            return None
        return math.sqrt(self.S / (self.k-2))

with sqlite3.connect(':memory:') as con:

    con.create_aggregate("stdev", 1, StdevFunc)

    cur = con.cursor()

    cur.execute("create table test(i)")
    cur.executemany("insert into test(i) values (?)", [(1,), (2,), (3,), (4,), (5,)])
    cur.execute("insert into test(i) values (null)")
    cur.execute("select avg(i) from test")
    print("avg: %f" % cur.fetchone()[0])
    cur.execute("select stdev(i) from test")
    print("stdev: %f" % cur.fetchone()[0])

This will print:

avg: 3.000000
stdev: 1.581139

Compare with MySQL: http://sqlfiddle.com/#!2/ad42f3/3/0