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VAR_POP function [Aggregate]

Computes the statistical variance of a population consisting of a numeric-expression, as a DOUBLE.

VAR_POP( numeric-expression )

VAR_POP( numeric-expression ) OVER ( window-spec )

window-spec : see Syntax 2 instructions in the Usage section below

numeric-expression    The expression whose population-based variance is calculated over a set of rows. The expression is commonly a column name.

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 (s²) of numeric-expression (x) is computed according to the following formula:

s² = (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.

• Aggregate functions

• SQL/2003    SQL foundation feature (T611) outside of core SQL.

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;