RDBMS Type: MySQL
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.
This function executes as a window function if over_clause is present.
over_clause is as described in