harmonic mean in SQL

[Ted Yu]

Hi,
The mathematical way to add two "partial" harmonic means to generate new
harmonic mean is:

Let X1  , X2 be two harmonic means on different rollup rows, generated using
n1 and n2 # of facts respectively.

The combined harmonic mean would be:

(n1 + n2)/( n1/x1 + n2/x2)

If you have experience with computing harmonic mean in SQL, please share.