corr(x,y) - Returns the Pearson coefficient of correlation
between a set of number pairs
The function takes as arguments any pair of numeric types and returns a double.
Any pair with a NULL is ignored. If the function is applied to an empty set or
a singleton set, NULL will be returned. Otherwise, it computes the following:
   COVAR_POP(x,y)/(STDDEV_POP(x)*STDDEV_POP(y))
where neither x nor y is null,
COVAR_POP is the population covariance,
and STDDEV_POP is the population standard deviation.