# VAR_POP function

VAR_POP is an aggregate function that evaluates the population variance of an expression over a set of rows.

See Aggregates (set functions) for more information about these functions.

VAR_POP is allowed only on expressions that evaluate to numeric data types.

## Syntax

    VAR_POP ( expression )

The expression can contain multiple column references or expressions, but it cannot contain another aggregate or subquery. It must evaluate to a built-in numeric data type. If an expression evaluates to NULL, the aggregate skips that value.

The resulting data type is DOUBLE (it might overflow).

## Formula

Population variance is calculated as follows:

    sum(xi2)/n - m2

    where

    n is the number of items in the population
    m is the population average
    x1 ... xn are the items in the population

## Example

    -- find the variance in flight time per aircraft
    SELECT AIRCRAFT, VAR_POP( flying_time )
    FROM flights GROUP BY aircraft;

Parent topic: Built-in functions