Returns the statistical standard deviation of all values in number-expression, evaluated for each row remaining in the group and defined as the square root of the sample variance (VAR_SAMP).
STDDEV_SAMP ( [DISTINCT | ALL] number-expression )
where ALL includes (and DISTINCT excludes) duplicate values in the input set. ALL is the default. When the the input set has no non-null data, STDDEV_SAMP returns NULL.
STD_DEV is an alias of STDDEV_SAMP.