Syntax:
STD(expr) STDDEV(expr)

Returns the population standard deviation of expr. This is an extension
to standard SQL. The STDDEV() form of this function is provided for
compatibility with Oracle. As of MySQL 5.0.3, the standard SQL function
STDDEV_POP() can be used instead.

These functions return NULL if there were no matching rows.

URL: http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html

