This function converts its arguments to DOUBLE, and performs the computation in double-precision floating-point arithmetic.
If the function is applied to an empty set, then it returns NULL.
Both dependent-expression and independent-expression are numeric. The function is applied to the set of (dependent-expression, independent-expression) after eliminating the pairs for which either dependent-expression or independent-expression is NULL. The following computation is made:
COVAR_POP ( y, x ) / STDDEV_POP ( y ) * STDDEV_POP ( x )
where y represents the dependent-expression and x represents the independent-expression.