Syntax:
VAR_POP(expr)

Returns the population standard variance of expr. It considers rows as
the whole population, not as a sample, so it has the number of rows as
the denominator. You can also use VARIANCE(), which is equivalent but
is not standard SQL.

If there are no matching rows, VAR_POP() returns NULL.

URL: http://dev.mysql.com/doc/refman/5.5/en/group-by-functions.html

