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 Remarks section below
numeric-expression The expression whose population-based variance is calculated over a set of rows. The expression is commonly a column name.
DOUBLE
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.
SQL/2003 SQL foundation feature (T611) outside 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; |
Year | Quarter | Average | Variance |
---|---|---|---|
2000 | 1 | 25.775148 | 203.9021... |
2000 | 2 | 27.050847 | 225.8109... |
... | ... | ... | ... |
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