Is there a STDDEV equivalent in Cloud Spanner functions?

Question

I don't see it in the documentation; is it perhaps part of an existing function I didn't notice, or available in some other way?

Answer 1

As Andrei Tigau mentioned, STDDEV is not supported yet. That said, you need to calculate it in two pass. Assuming you are interested in column x of YourTable,

SELECT SQRT(SUM(POW(x - avg, 2)/(n-1)))
FROM (SELECT AVG(x) AS avg, COUNT(*) AS n FROM YourTable)
  CROSS JOIN YourTable;

You may try following one pass solution as well.

SELECT SQRT(s2/(n-1) - POW(s/n, 2))
FROM (
  SELECT COUNT(*) AS n, SUM(x) AS s, SUM(x*x) AS s2
  FROM YourTable
);

Depending on type, you may have to cast it to double (especially for s2) to avoid overflow. Both will suffer from floating point errors.