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### STDDEV_SAMP function [Aggregate]

Computes the standard deviation of a sample consisting of a numeric-expression, as a DOUBLE.

###### Syntax 1

STDDEV_SAMP( numeric-expression )

###### Syntax 2

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

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

###### Parameters

numeric-expression    The expression whose sample-based standard deviation is calculated over a set of rows. The expression is commonly a column name.

###### Remarks

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

The standard deviation (s) is computed according to the following formula, which assumes a normal distribution:

s = [ (1/( N - 1 )) * SUM( xi - mean( x ) )2 ]1/2

This standard deviation does not include rows where numeric-expression is NULL. It returns NULL for a group containing either 0 or 1 rows.

For more information about the statistical computation performed, see Mathematical formulas for the aggregate functions.

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.

###### Standards and compatibility
• SQL/2003    SQL foundation feature (T621) outside of core SQL.

###### Example

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,
STDDEV_SAMP( quantity ) AS Variance
FROM SalesOrderItems
GROUP BY Year, Quarter
ORDER BY Year, Quarter;```
YearQuarterAverageVariance
2000125.77514814.3218...
2000227.05084715.0696...
............