Syntax:
STDDEV_POP(expr)

Returns the population standard deviation of expr (the square root of
VAR_POP()). You can also use STD() or STDDEV(), which are equivalent
but not standard SQL.

STDDEV_POP() returns NULL if there were no matching rows.

URL: http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html

