VAR_SAMP function

VAR_SAMP is an aggregate function that evaluates the sample variance of an expression over a set of rows.

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

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

Syntax

VAR_SAMP ( 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

Sample variance is calculated as follows:


[ sum(x_i²) - sum(x_i)²/n ]/(n-1)

where

n is the number of items in the population
x₁ ... x_n are the items in the population

Example

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