This function converts its argument to DOUBLE, performs the computation in double-precision floating point, and returns a DOUBLE as the result.

The population-based variance (s^{2}) of numeric-expression (x) is computed according to the following formula:

s^{2} = (1/N) * SUM( x_{i} - mean( x ) )^{2}

This variance does not include rows where numeric-expression is NULL. It returns NULL for a group containing no rows.

Syntax 2 represents usage as a window function in a SELECT statement. As such, elements of window-spec can be specified either in the function syntax (inline), or in conjunction with a WINDOW clause in the SELECT statement. See the window-spec definition provided in WINDOW clause.

For more information about using window functions in SELECT statements, including working examples, see Window functions.

The following statement lists the average and variance in the number of items per order in different time periods:

SELECT YEAR( ShipDate ) AS Year,
QUARTER( ShipDate ) AS Quarter,
AVG( Quantity ) AS Average,
VAR_POP( quantity ) AS Variance
FROM SalesOrderItems
GROUP BY Year, Quarter
ORDER BY Year, Quarter;