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. This function was added in MySQL 5.0.3. Before 5.0.3,
you can use VARIANCE(), which is equivalent but is not standard SQL.

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